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package smtp

import ""

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

It also implements the following extensions:

AUTH: RFC 2554

LMTP (RFC 2033) is also supported.

Additional extensions may be handled by other packages.



Package Files

backend.go client.go conn.go data.go lengthlimit_reader.go parse.go server.go smtp.go


var (
    ErrAuthRequired    = errors.New("Please authenticate first")
    ErrAuthUnsupported = errors.New("Authentication not supported")
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).

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.

var ErrDataTooLarge = &SMTPError{
    Code:         552,
    EnhancedCode: EnhancedCode{5, 3, 4},
    Message:      "Maximum message size exceeded",
var ErrTooLongLine = errors.New("smtp: too longer line in input stream")
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.

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 if possible, authenticates with the optional mechanism a if possible, and then sends an email from address from, to addresses to, with message r. The addr must include a port, as in "".

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.



// Set up authentication information.
auth := sasl.NewPlainClient("", "", "password")

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

type Backend

type Backend interface {
    // Authenticate a user. Return smtp.ErrAuthUnsupported if you don't want to
    // support this.
    Login(state *ConnectionState, username, password string) (Session, error)

    // Called if the client attempts to send mail without logging in first.
    // Return smtp.ErrAuthRequired if you don't want to support this.
    AnonymousLogin(state *ConnectionState) (Session, error)

A SMTP server backend.

type BodyType

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

type Client

type Client struct {
    // Text is the textproto.Conn used by the Client. It is exported to allow for
    // clients to add extensions.
    Text *textproto.Conn

    // 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 "".



// Connect to the remote SMTP server.
c, err := smtp.Dial("")
if err != nil {

// Set the sender and recipient first
if err := c.Mail("", nil); err != nil {
if err := c.Rcpt(""); err != nil {

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

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

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 "".

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

func NewClient

func NewClient(conn net.Conn, host string) (*Client, error)

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, host string) (*Client, error)

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

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) 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) 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 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) StartTLS

func (c *Client) StartTLS(config *tls.Config) error

StartTLS sends the STARTTLS command and encrypts all further communication. Only servers that advertise the STARTTLS extension support this function.

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

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

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) ReadLine

func (c *Conn) ReadLine() (string, error)

Reads a line of input

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) SetSession

func (c *Conn) SetSession(session Session)

Setting the user resets any message being generated

func (*Conn) State

func (c *Conn) State() ConnectionState

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.

func (*Conn) WriteResponse

func (c *Conn) WriteResponse(code int, enhCode EnhancedCode, text ...string)

type ConnectionState

type ConnectionState struct {
    Hostname   string
    LocalAddr  net.Addr
    RemoteAddr net.Addr
    TLS        tls.ConnectionState

type EnhancedCode

type EnhancedCode [3]int

type LMTPSession

type LMTPSession interface {
    // 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 int

    // 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

    // 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 custom arguments that were passed as an argument to the MAIL 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 SaslServerFactory

type SaslServerFactory func(conn *Conn) sasl.Server

A function that creates SASL servers.

type Server

type Server struct {
    // 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 mode cannot be used with a
    // TCP listener.
    LMTP bool

    Domain            string
    MaxRecipients     int
    MaxMessageBytes   int
    MaxLineLength     int
    AllowInsecureAuth bool
    Strict            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

    // If set, the AUTH command will not be advertised and authentication
    // attempts will be rejected. This setting overrides AllowInsecureAuth.
    AuthDisabled bool

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

A SMTP server.

func NewServer

func NewServer(be Backend) *Server

New creates a new SMTP server.



be := &Backend{}

s := smtp.NewServer(be)

s.Addr = ":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 {

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) EnableAuth

func (s *Server) EnableAuth(name string, f SaslServerFactory)

EnableAuth enables an authentication mechanism on this server.

This function should not be called directly, it must only be used by libraries implementing extensions of the SMTP protocol.

func (*Server) ForEachConn

func (s *Server) ForEachConn(f func(*Conn))

ForEachConn iterates through all opened connections.

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, ":smtps" is used.

func (*Server) Serve

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

Serve accepts incoming connections on the Listener l.

type Session

type Session interface {
    // Discard currently processed message.

    // 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) error
    // Set currently processed message contents and send it.
    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.


backendutilPackage backendutil provide utilities to implement SMTP backends.

Package smtp imports 18 packages (graph).

Version v0.15.0 (latest) | Published Mar 24, 2021 | Platform: linux/amd64 | Updated 3 weeks ago

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