smtp

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 Go to latest Published: Aug 26, 2024 License: MIT Imports: 19 Imported by: 0

README

go-smtp

Go Reference

An ESMTP client and server library written in Go.

Features

  • ESMTP client & server implementing RFC 5321
  • Support for additional SMTP extensions such as AUTH and PIPELINING
  • UTF-8 support for subject and message
  • LMTP support

Relationship with net/smtp

The Go standard library provides a SMTP client implementation in net/smtp. However net/smtp is frozen: it's not getting any new features. go-smtp provides a server implementation and a number of client improvements.

Licence

MIT

Documentation

Overview

Package smtp implements the Simple Mail Transfer Protocol as defined in RFC 5321.

It also implements the following extensions:

LMTP (RFC 2033) is also supported.

Additional extensions may be handled by other packages.

Index

Examples

Constants

This section is empty.

Variables

View Source 
var (
	ErrAuthFailed = &SMTPError{
		Code:         535,
		EnhancedCode: EnhancedCode{5, 7, 8},
		Message:      "Authentication failed",
	}
	ErrAuthRequired = &SMTPError{
		Code:         502,
		EnhancedCode: EnhancedCode{5, 7, 0},
		Message:      "Please authenticate first",
	}
	ErrAuthUnsupported = &SMTPError{
		Code:         502,
		EnhancedCode: EnhancedCode{5, 7, 0},
		Message:      "Authentication not supported",
	}
	ErrAuthUnknownMechanism = &SMTPError{
		Code:         504,
		EnhancedCode: EnhancedCode{5, 7, 4},
		Message:      "Unsupported authentication mechanism",
	}
)
View Source 
var EnhancedCodeNotSet = EnhancedCode{0, 0, 0}

EnhancedCodeNotSet is a nil value of EnhancedCode field in SMTPError, used to indicate that backend failed to provide enhanced status code. X.0.0 will be used (X is derived from error code).

View Source 
var ErrDataReset = errors.New("smtp: message transmission aborted")

ErrDataReset is returned by Reader pased to Data function if client does not send another BDAT command and instead closes connection or issues RSET command.

View Source 
var ErrDataTooLarge = &SMTPError{
	Code:         552,
	EnhancedCode: EnhancedCode{5, 3, 4},
	Message:      "Maximum message size exceeded",
}
View Source 
var ErrServerClosed = errors.New("smtp: server already closed")
View Source 
var ErrTooLongLine = errors.New("smtp: too long a line in input stream")
View Source 
var NoEnhancedCode = EnhancedCode{-1, -1, -1}

NoEnhancedCode is used to indicate that enhanced error code should not be included in response.

Note that RFC 2034 requires an enhanced code to be included in all 2xx, 4xx and 5xx responses. This constant is exported for use by extensions, you should probably use EnhancedCodeNotSet instead.

Functions

func SendMail 

func SendMail(addr string, a sasl.Client, from string, to []string, r io.Reader) error

SendMail connects to the server at addr, switches to TLS, authenticates with the optional SASL client, and then sends an email from address from, to addresses to, with message r. The addr must include a port, as in "mail.example.com:smtp".

The addresses in the to parameter are the SMTP RCPT addresses.

The r parameter should be an RFC 822-style email with headers first, a blank line, and then the message body. The lines of r should be CRLF terminated. The r headers should usually include fields such as "From", "To", "Subject", and "Cc". Sending "Bcc" messages is accomplished by including an email address in the to parameter but not including it in the r headers.

SendMail is intended to be used for very simple use-cases. If you want to customize SendMail's behavior, use a Client instead.

The SendMail function and the go-smtp package are low-level mechanisms and provide no support for DKIM signing (see go-msgauth), MIME attachments (see the mime/multipart package or the go-message package), or other mail functionality.

Example 
// Set up authentication information.
auth := sasl.NewPlainClient("", "user@example.com", "password")

// Connect to the server, authenticate, set the sender and recipient,
// and send the email all in one step.
to := []string{"recipient@example.net"}
msg := strings.NewReader("To: recipient@example.net\r\n" +
	"Subject: discount Gophers!\r\n" +
	"\r\n" +
	"This is the email body.\r\n")
err := smtp.SendMail("mail.example.com:25", auth, "sender@example.org", to, msg)
if err != nil {
	log.Fatal(err)
}

Output:
Example (PlainAuth) 
// hostname is used by PlainAuth to validate the TLS certificate.
hostname := "mail.example.com"
auth := sasl.NewPlainClient("", "user@example.com", "password")

err := smtp.SendMail(hostname+":25", auth, from, recipients, msg)
if err != nil {
	log.Fatal(err)
}

Output:

func SendMailTLS 

func SendMailTLS(addr string, a sasl.Client, from string, to []string, r io.Reader) error

SendMailTLS works like SendMail, but with implicit TLS.

Types

type AuthSession 

type AuthSession interface {
	Session

	AuthMechanisms() []string
	Auth(mech string) (sasl.Server, error)
}

AuthSession is an add-on interface for Session. It provides support for the AUTH extension.

type Backend 

type Backend interface {
	NewSession(c *Conn) (Session, error)
}

A SMTP server backend.

type BackendFunc 

type BackendFunc func(c *Conn) (Session, error)

BackendFunc is an adapter to allow the use of an ordinary function as a Backend.

func (BackendFunc) NewSession 

func (f BackendFunc) NewSession(c *Conn) (Session, error)

NewSession calls f(c).

type BodyType 

type BodyType string
const (
	Body7Bit       BodyType = "7BIT"
	Body8BitMIME   BodyType = "8BITMIME"
	BodyBinaryMIME BodyType = "BINARYMIME"
)

type Client 

type Client struct {

	// Time to wait for command responses (this includes 3xx reply to DATA).
	CommandTimeout time.Duration
	// Time to wait for responses after final dot.
	SubmissionTimeout time.Duration

	// Logger for all network activity.
	DebugWriter io.Writer
	// contains filtered or unexported fields
}

A Client represents a client connection to an SMTP server.

func Dial 

func Dial(addr string) (*Client, error)

Dial returns a new Client connected to an SMTP server at addr. The addr must include a port, as in "mail.example.com:smtp".

This function returns a plaintext connection. To enable TLS, use DialStartTLS.

Example 
// Connect to the remote SMTP server.
c, err := smtp.Dial("mail.example.com:25")
if err != nil {
	log.Fatal(err)
}

// Set the sender and recipient first
if err := c.Mail("sender@example.org", nil); err != nil {
	log.Fatal(err)
}
if err := c.Rcpt("recipient@example.net", nil); err != nil {
	log.Fatal(err)
}

// Send the email body.
wc, err := c.Data()
if err != nil {
	log.Fatal(err)
}
_, err = fmt.Fprintf(wc, "This is the email body")
if err != nil {
	log.Fatal(err)
}
err = wc.Close()
if err != nil {
	log.Fatal(err)
}

// Send the QUIT command and close the connection.
err = c.Quit()
if err != nil {
	log.Fatal(err)
}

Output:

func DialStartTLS 

func DialStartTLS(addr string, tlsConfig *tls.Config) (*Client, error)

DialStartTLS retruns a new Client connected to an SMTP server via STARTTLS at addr. The addr must include a port, as in "mail.example.com:smtp".

A nil tlsConfig is equivalent to a zero tls.Config.

func DialTLS 

func DialTLS(addr string, tlsConfig *tls.Config) (*Client, error)

DialTLS returns a new Client connected to an SMTP server via TLS at addr. The addr must include a port, as in "mail.example.com:smtps".

A nil tlsConfig is equivalent to a zero tls.Config.

func NewClient 

func NewClient(conn net.Conn) *Client

NewClient returns a new Client using an existing connection and host as a server name to be used when authenticating.

func NewClientLMTP 

func NewClientLMTP(conn net.Conn) *Client

NewClientLMTP returns a new LMTP Client (as defined in RFC 2033) using an existing connection and host as a server name to be used when authenticating.

func NewClientStartTLS 

func NewClientStartTLS(conn net.Conn, tlsConfig *tls.Config) (*Client, error)

NewClientStartTLS creates a new Client and performs a STARTTLS command.

func (*Client) Auth 

func (c *Client) Auth(a sasl.Client) error

Auth authenticates a client using the provided authentication mechanism. Only servers that advertise the AUTH extension support this function.

If server returns an error, it will be of type *SMTPError.

func (*Client) Close 

func (c *Client) Close() error

Close closes the connection.

func (*Client) Data 

func (c *Client) Data() (io.WriteCloser, error)

Data issues a DATA command to the server and returns a writer that can be used to write the mail headers and body. The caller should close the writer before calling any more methods on c. A call to Data must be preceded by one or more calls to Rcpt.

If server returns an error, it will be of type *SMTPError.

func (*Client) Extension 

func (c *Client) Extension(ext string) (bool, string)

Extension reports whether an extension is support by the server. The extension name is case-insensitive. If the extension is supported, Extension also returns a string that contains any parameters the server specifies for the extension.

func (*Client) Hello 

func (c *Client) Hello(localName string) error

Hello sends a HELO or EHLO to the server as the given host name. Calling this method is only necessary if the client needs control over the host name used. The client will introduce itself as "localhost" automatically otherwise. If Hello is called, it must be called before any of the other methods.

If server returns an error, it will be of type *SMTPError.

func (*Client) LMTPData 

func (c *Client) LMTPData(statusCb func(rcpt string, status *SMTPError)) (io.WriteCloser, error)

LMTPData is the LMTP-specific version of the Data method. It accepts a callback that will be called for each status response received from the server.

Status callback will receive a SMTPError argument for each negative server reply and nil for each positive reply. I/O errors will not be reported using callback and instead will be returned by the Close method of io.WriteCloser. Callback will be called for each successfull Rcpt call done before in the same order.

func (*Client) Mail 

func (c *Client) Mail(from string, opts *MailOptions) error

Mail issues a MAIL command to the server using the provided email address. If the server supports the 8BITMIME extension, Mail adds the BODY=8BITMIME parameter. This initiates a mail transaction and is followed by one or more Rcpt calls.

If opts is not nil, MAIL arguments provided in the structure will be added to the command. Handling of unsupported options depends on the extension.

If server returns an error, it will be of type *SMTPError.

func (*Client) MaxMessageSize 

func (c *Client) MaxMessageSize() (size int, ok bool)

MaxMessageSize returns the maximum message size accepted by the server. 0 means unlimited.

If the server doesn't convey this information, ok = false is returned.

func (*Client) Noop 

func (c *Client) Noop() error

Noop sends the NOOP command to the server. It does nothing but check that the connection to the server is okay.

func (*Client) Quit 

func (c *Client) Quit() error

Quit sends the QUIT command and closes the connection to the server.

If Quit fails the connection is not closed, Close should be used in this case.

func (*Client) Rcpt 

func (c *Client) Rcpt(to string, opts *RcptOptions) error

Rcpt issues a RCPT command to the server using the provided email address. A call to Rcpt must be preceded by a call to Mail and may be followed by a Data call or another Rcpt call.

If opts is not nil, RCPT arguments provided in the structure will be added to the command. Handling of unsupported options depends on the extension.

If server returns an error, it will be of type *SMTPError.

func (*Client) Reset 

func (c *Client) Reset() error

Reset sends the RSET command to the server, aborting the current mail transaction.

func (*Client) SendMail 

func (c *Client) SendMail(from string, to []string, r io.Reader) error

SendMail will use an existing connection to send an email from address from, to addresses to, with message r.

This function does not start TLS, nor does it perform authentication. Use DialStartTLS and Auth before-hand if desirable.

The addresses in the to parameter are the SMTP RCPT addresses.

The r parameter should be an RFC 822-style email with headers first, a blank line, and then the message body. The lines of r should be CRLF terminated. The r headers should usually include fields such as "From", "To", "Subject", and "Cc". Sending "Bcc" messages is accomplished by including an email address in the to parameter but not including it in the r headers.

func (*Client) SupportsAuth 

func (c *Client) SupportsAuth(mech string) bool

SupportsAuth checks whether an authentication mechanism is supported.

func (*Client) TLSConnectionState 

func (c *Client) TLSConnectionState() (state tls.ConnectionState, ok bool)

TLSConnectionState returns the client's TLS connection state. The return values are their zero values if STARTTLS did not succeed.

func (*Client) Verify 

func (c *Client) Verify(addr string) error

Verify checks the validity of an email address on the server. If Verify returns nil, the address is valid. A non-nil return does not necessarily indicate an invalid address. Many servers will not verify addresses for security reasons.

If server returns an error, it will be of type *SMTPError.

type Conn 

type Conn struct {
	// contains filtered or unexported fields
}

func (*Conn) Close 

func (c *Conn) Close() error

func (*Conn) Conn 

func (c *Conn) Conn() net.Conn

func (*Conn) Hostname 

func (c *Conn) Hostname() string

func (*Conn) Reject 

func (c *Conn) Reject()

func (*Conn) Server 

func (c *Conn) Server() *Server

func (*Conn) Session 

func (c *Conn) Session() Session

func (*Conn) TLSConnectionState 

func (c *Conn) TLSConnectionState() (state tls.ConnectionState, ok bool)

TLSConnectionState returns the connection's TLS connection state. Zero values are returned if the connection doesn't use TLS.

type DSNAddressType 

type DSNAddressType string
const (
	DSNAddressTypeRFC822 DSNAddressType = "RFC822"
	DSNAddressTypeUTF8   DSNAddressType = "UTF-8"
)

type DSNNotify 

type DSNNotify string
const (
	DSNNotifyNever   DSNNotify = "NEVER"
	DSNNotifyDelayed DSNNotify = "DELAY"
	DSNNotifyFailure DSNNotify = "FAILURE"
	DSNNotifySuccess DSNNotify = "SUCCESS"
)

type DSNReturn 

type DSNReturn string
const (
	DSNReturnFull    DSNReturn = "FULL"
	DSNReturnHeaders DSNReturn = "HDRS"
)

type EnhancedCode 

type EnhancedCode [3]int

type LMTPSession 

type LMTPSession interface {
	Session

	// LMTPData is the LMTP-specific version of Data method.
	// It can be optionally implemented by the backend to provide
	// per-recipient status information when it is used over LMTP
	// protocol.
	//
	// LMTPData implementation sets status information using passed
	// StatusCollector by calling SetStatus once per each AddRcpt
	// call, even if AddRcpt was called multiple times with
	// the same argument. SetStatus must not be called after
	// LMTPData returns.
	//
	// Return value of LMTPData itself is used as a status for
	// recipients that got no status set before using StatusCollector.
	LMTPData(r io.Reader, status StatusCollector) error
}

LMTPSession is an add-on interface for Session. It can be implemented by LMTP servers to provide extra functionality.

type Logger 

type Logger interface {
	Printf(format string, v ...interface{})
	Println(v ...interface{})
}

Logger interface is used by Server to report unexpected internal errors.

type MailOptions 

type MailOptions struct {
	// Value of BODY= argument, 7BIT, 8BITMIME or BINARYMIME.
	Body BodyType

	// Size of the body. Can be 0 if not specified by client.
	Size int64

	// TLS is required for the message transmission.
	//
	// The message should be rejected if it can't be transmitted
	// with TLS.
	RequireTLS bool

	// The message envelope or message header contains UTF-8-encoded strings.
	// This flag is set by SMTPUTF8-aware (RFC 6531) client.
	UTF8 bool

	// Value of RET= argument, FULL or HDRS.
	Return DSNReturn

	// Envelope identifier set by the client.
	EnvelopeID string

	// Accepted Domain from Exchange Online, e.g. from OutgoingConnector
	XOORG string

	// The authorization identity asserted by the message sender in decoded
	// form with angle brackets stripped.
	//
	// nil value indicates missing AUTH, non-nil empty string indicates
	// AUTH=<>.
	//
	// Defined in RFC 4954.
	Auth *string
}

MailOptions contains parameters for the MAIL command.

type RcptOptions 

type RcptOptions struct {
	// Value of NOTIFY= argument, NEVER or a combination of either of
	// DELAY, FAILURE, SUCCESS.
	Notify []DSNNotify

	// Original recipient set by client.
	OriginalRecipientType DSNAddressType
	OriginalRecipient     string
}

RcptOptions contains parameters for the RCPT command.

type SMTPError 

type SMTPError struct {
	Code         int
	EnhancedCode EnhancedCode
	Message      string
}

SMTPError specifies the error code, enhanced error code (if any) and message returned by the server.

func (*SMTPError) Error 

func (err *SMTPError) Error() string

func (*SMTPError) Temporary 

func (err *SMTPError) Temporary() bool

type Server 

type Server struct {
	// The type of network, "tcp" or "unix".
	Network string
	// TCP or Unix address to listen on.
	Addr string
	// The server TLS configuration.
	TLSConfig *tls.Config
	// Enable LMTP mode, as defined in RFC 2033.
	LMTP bool

	Domain            string
	MaxRecipients     int
	MaxMessageBytes   int64
	MaxLineLength     int
	AllowInsecureAuth bool
	Debug             io.Writer
	ErrorLog          Logger
	ReadTimeout       time.Duration
	WriteTimeout      time.Duration

	// Advertise SMTPUTF8 (RFC 6531) capability.
	// Should be used only if backend supports it.
	EnableSMTPUTF8 bool

	// Advertise REQUIRETLS (RFC 8689) capability.
	// Should be used only if backend supports it.
	EnableREQUIRETLS bool

	// Advertise BINARYMIME (RFC 3030) capability.
	// Should be used only if backend supports it.
	EnableBINARYMIME bool

	// Advertise DSN (RFC 3461) capability.
	// Should be used only if backend supports it.
	EnableDSN bool

	// Advertise XOORG capability.
	// Should be used only if backend supports it.
	EnableXOORG bool

	// The server backend.
	Backend Backend
	// contains filtered or unexported fields
}

A SMTP server.

Example

ExampleServer runs an example SMTP server.

It can be tested manually with e.g. netcat: 

> netcat -C localhost 1025
EHLO localhost
AUTH PLAIN
AHVzZXJuYW1lAHBhc3N3b3Jk
MAIL FROM:<root@nsa.gov>
RCPT TO:<root@gchq.gov.uk>
DATA
Hey <3
.

package main

import (
	"errors"
	"io"
	"log"
	"time"

	"github.com/UPONU-GmbH/go-smtp"
	"github.com/emersion/go-sasl"
)

// The Backend implements SMTP server methods.
type Backend struct{}

// NewSession is called after client greeting (EHLO, HELO).
func (bkd *Backend) NewSession(c *smtp.Conn) (smtp.Session, error) {
	return &Session{}, nil
}

// A Session is returned after successful login.
type Session struct{}

// AuthMechanisms returns a slice of available auth mechanisms; only PLAIN is
// supported in this example.
func (s *Session) AuthMechanisms() []string {
	return []string{sasl.Plain}
}

// Auth is the handler for supported authenticators.
func (s *Session) Auth(mech string) (sasl.Server, error) {
	return sasl.NewPlainServer(func(identity, username, password string) error {
		if username != "username" || password != "password" {
			return errors.New("Invalid username or password")
		}
		return nil
	}), nil
}

func (s *Session) Mail(from string, opts *smtp.MailOptions) error {
	log.Println("Mail from:", from)
	return nil
}

func (s *Session) Rcpt(to string, opts *smtp.RcptOptions) error {
	log.Println("Rcpt to:", to)
	return nil
}

func (s *Session) Data(r io.Reader) error {
	if b, err := io.ReadAll(r); err != nil {
		return err
	} else {
		log.Println("Data:", string(b))
	}
	return nil
}

func (s *Session) Reset() {}

func (s *Session) Logout() error {
	return nil
}

// ExampleServer runs an example SMTP server.
//
// It can be tested manually with e.g. netcat:
//
//	> netcat -C localhost 1025
//	EHLO localhost
//	AUTH PLAIN
//	AHVzZXJuYW1lAHBhc3N3b3Jk
//	MAIL FROM:<root@nsa.gov>
//	RCPT TO:<root@gchq.gov.uk>
//	DATA
//	Hey <3
//	.
func main() {
	be := &Backend{}

	s := smtp.NewServer(be)

	s.Addr = "localhost:1025"
	s.Domain = "localhost"
	s.WriteTimeout = 10 * time.Second
	s.ReadTimeout = 10 * time.Second
	s.MaxMessageBytes = 1024 * 1024
	s.MaxRecipients = 50
	s.AllowInsecureAuth = true

	log.Println("Starting server at", s.Addr)
	if err := s.ListenAndServe(); err != nil {
		log.Fatal(err)
	}
}

Output:

func NewServer 

func NewServer(be Backend) *Server

New creates a new SMTP server.

func (*Server) Close 

func (s *Server) Close() error

Close immediately closes all active listeners and connections.

Close returns any error returned from closing the server's underlying listener(s).

func (*Server) ListenAndServe 

func (s *Server) ListenAndServe() error

ListenAndServe listens on the network address s.Addr and then calls Serve to handle requests on incoming connections.

If s.Addr is blank and LMTP is disabled, ":smtp" is used.

func (*Server) ListenAndServeTLS 

func (s *Server) ListenAndServeTLS() error

ListenAndServeTLS listens on the TCP network address s.Addr and then calls Serve to handle requests on incoming TLS connections.

If s.Addr is blank and LMTP is disabled, ":smtps" is used.

func (*Server) Serve 

func (s *Server) Serve(l net.Listener) error

Serve accepts incoming connections on the Listener l.

func (*Server) Shutdown 

func (s *Server) Shutdown(ctx context.Context) error

Shutdown gracefully shuts down the server without interrupting any active connections. Shutdown works by first closing all open listeners and then waiting indefinitely for connections to return to idle and then shut down. If the provided context expires before the shutdown is complete, Shutdown returns the context's error, otherwise it returns any error returned from closing the Server's underlying Listener(s).

type Session 

type Session interface {
	// Discard currently processed message.
	Reset()

	// Free all resources associated with session.
	Logout() error

	// Set return path for currently processed message.
	Mail(from string, opts *MailOptions) error
	// Add recipient for currently processed message.
	Rcpt(to string, opts *RcptOptions) error
	// Set currently processed message contents and send it.
	//
	// r must be consumed before Data returns.
	Data(r io.Reader) error
}

Session is used by servers to respond to an SMTP client.

The methods are called when the remote client issues the matching command.

type StatusCollector 

type StatusCollector interface {
	SetStatus(rcptTo string, err error)
}

StatusCollector allows a backend to provide per-recipient status information.

Source Files

View all Source files

Directories

Path Synopsis
cmd
smtp-debug-server

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