SMTP Server

8.12

SMTP Server🔗ℹ

This module module provides a minimal implementation of RFC 5321 [RFC5321] that can receive e-mail messages.

1 Usage🔗ℹ

(define stop
(start-smtp-server println))

The example above starts an SMTP server on localhost port 25 that prints all incoming e-mail to standard out. Calling the stop function terminates any connections in flight and stops the server.

See "example/" in the source code repository for an example with STARTTLS support.

2 Reference🔗ℹ

value
tls-encode-proc/c :
(-> input-port?
    output-port?
    #:mode 'tcp
    #:encrypt 'tls
    #:close-original? #t
    (values input-port? output-port?))

The contract for TLS-encoding procedures. See also ports->ssl-ports.

parameter
(current-smtp-hostname) → non-empty-string?
(current-smtp-hostname hostname) → void?
hostname : non-empty-string?
= (gethostname)

Controls the host name displayed to clients.

struct
(struct envelope (sender recipients data)
    #:extra-constructor-name make-envelope)
  sender : bytes?
  recipients : (listof bytes?)
  data : bytes?

Represents the sender, recipients and contents of a receieved e-mail.

procedure
(start-smtp-server handler
[ #:host host
#:port port
#:limits lim
#:tls-encode tls-encode]) → (-> void?)
  handler : (-> envelope? void?)
  host : string? = "127.0.0.1"
  port : (integer-in 0 65535) = 25
  lim : smtp-limits? = (make-smtp-limits)
  tls-encode : (or/c #f tls-encode-proc/c) = #f

Starts an SMTP server that listens on host and port and returns a function that will stop the server when applied.

Successfully-received e-mails are passed to handler. When the handler raises an exception, the server notifies the client that the message has been rejected.

The #:limits arguments can be used to customize various security limits.

If the optional #:tls-encode argument supplies a tls-encode-proc/c value, the server advertises STARTTLS support and clients may opt in to TLS encryption.

procedure
(smtp-limits? v) → boolean?
  v : any/c
procedure
(make-smtp-limits [ #:max-connections max-connections
#:max-line-length max-line-length
#:max-envelope-length max-envelope-length
#:session-timeout session-timeout])
→ smtp-limits?
  max-connections : exact-positive-integer? = 512
  max-line-length : exact-nonnegative-integer? = 1024
   max-envelope-length : exact-nonnegative-integer?
= (* 10 1024 1024)
  session-timeout : (and/c number? positive?) = 300

Security limits allow you to configure various security-related limits on an SMTP server.

The #:max-connections argument controls the maximum number of concurrent client connections that the server will accept at a time.

The #:max-line-length argument controls the maximum length in bytes of each line received from a client may be. The server will reject lines longer than this amount.

The #:max-envelope-length argument controls the maximum length of incoming e-mails from clients. The total length of an envlope includes the length in bytes of the sender and the recipients list as well as the message data.

The #:session-timeout argument controls the maximum amount of time, in seconds, that a client session may be open for.

Bibliography🔗ℹ

[RFC5321]

J. Klensin, “Simple Mail Transfer Protocol.” 2008. https://www.ietf.org/rfc/rfc5321.html