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Module Netsmtp

module Netsmtp: sig .. end

This is an interface for the Simple Mail Tranfer Protocol (SMTP) as specified by RFC 2821.

exception Protocol_error
exception Authentication_error
exception Transient_error of int * string
exception Permanent_error of int * string
val tcp_port : int

default TCP port for SMTP

The class client implements the SMTP protocol. Client objects are created by

 new client in_ch out_ch

where in_ch is an input channel representing the input direction of the TCP stream, and where out_ch is an output channel representing the output direction of the TCP stream.

class client : Netchannels.in_obj_channel -> Netchannels.out_obj_channel -> object .. end
class connect : ?proxy:#Uq_engines.client_endpoint_connector -> Uq_engines.connect_address -> float -> client

connect addr timeout: Connects with the server at addr, and configure that I/O operations time out after timeout seconds of waiting.

val auth_mechanisms : string list -> string list

If applied to helo_response, returns the list of AUTH mechanisms

val authenticate : ?host:string ->
?tls_config:Netsys_crypto_types.tls_config ->
?tls_required:bool ->
?tls_peer:string ->
?sasl_mechs:Netsys_sasl.sasl_mechanism list ->
?sasl_params:(string * string * bool) list ->
?user:string ->
?authz:string -> ?creds:Netsys_sasl.credentials -> client -> unit

Authenticates the session:

  • sends the EHLO string
  • if the server supports TLS, and tls_config is set, the TLS session is started, and EHLO is repeated
  • if AUTH support is announced by the server, one of the sasl_mechs is taken and used for authentication. If sasl_mechs is empty, this authentication step is skipped.


  • host: the host name of the client
  • tls_config: if set, TLS is tried on the connection
  • tls_required: if set, it is even required that TLS is supported. If not, a Netsys_types.TLS_error exception is raised.
  • tls_peer: the host name of the server (only needed for TLS, and only needed if the TLS configuration authenticates the server, or if the SNI extension is active)
  • sasl_mechs: available SASL mechanisms (in order of preference). If you pass mechanisms, you'll normally also need to pass user and creds.
  • sasl_params: parameters for SASL. A "digest-uri" parameter is always generated, and need not to be set
  • user: the user name to authenticate as
  • authz: the identity to act as (authorization name)
  • creds: credentials

Regarding TLS: note that it is uncommon to require the server to be authenticated (opportunistic encryption), because servers often do not have certficicates from a regular trust center. You can get such a TLS config with

let tls_config =

SASL example:

  ~sasl_mechs:[ (module Netmech_scram_sasl.SCRAM_SHA1);
                (module Netmech_digest_sasl.DIGEST_MD5);
  ~creds:[ "password", "sEcReT", [] ]
val sendmail : client -> Netmime.complex_mime_message -> unit

Sends the email to the receivers in the to, cc, and bcc headers. The SMTP server must support relaying of emails. See also Netsendmail.sendmail.


module Debug: sig .. end
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