sendmail ~hostname ?port ~domain ~authenticator ?authentication sender recipients mail where:

• hostname is the hostname of the peer
• port the port of the SMTP peer
• domain is the domain of the sender (probably localhost)
• authenticator is the TLS authenticator
• authentication is the username and the password of the user
• sender is the sender
• recipients are recipients of the email
• mail stream of the mail

The connection already start a TLS connection to the peer. The peer is probably available on *:465 (the default of port argument). The mail stream must emit for each chunk a CRLF at the end (a line). As an user of GMail, the call of sendmail looks like:

open Mrmime

let my_domain = Colombe.Domain.of_string_exn (Unix.gethostname ())
let my_authentication =
  { Sendmail.username= "my_login"
  ; Sendmail.password= "my_password"
  ; Sendmail.mechanism= Sendmail.PLAIN }

let sender =
  let open Mrmime.Mailbox in
  let v = Local.[ "my"; "address"; "mail" ] @ Domain.(domain, [ "gmail"; "com" ]) in
  Result.get_ok (Colombe_emile.to_reverse_path v)
  (* "my.address.mail@gmail.com" *)

let destination =
  let open Mrmime.Mailbox in
  let v = Local.[ "to"; "joe" ] @ Domain.(domain, [ "gmail"; "com" ]) in
  Result.get_ok (Colombe_emile.to_forward_path v)
  (* "to.joe@gmail.com" *)

let run () = sendmail
  ~hostname:Domain_name.(host_exe (of_string_exn "gmail.com"))
  ~domain:my_domain
  ~authentictor
  ~authentication
  sender [ destination ]
  mail

let () = match Lwt_main.run (run ()) with
  | Ok () -> ()
  | Error err -> Format.eprintf "%a.\n%!" Sendmail.pp_error err

sendmail does not strictly depend on mrmime or emile. However, we advise to use them to produce typed and well-formed mails. sendmail does not handle properly contents of mails as are. It just emits the stream to the pipeline directly without any changes if the line does not start with a dot ("."). Otherwise, it prepends the line with a new dot (which has a signification in terms of SMTP).

We assume that each call of mail () gives to us a line - something which ends up with CRLF ("\r\n"). By this way, we can sanitize the dot character - and only on this way.

mrmime ensures to make on its way a stream which emits line per line. A non-user of mrmime should be aware about this assumption.

sendmail starts by itself a TLS connection with the SMTP server.

sendmail_with_starttls is sendmail but a part of the communication is insecure. Usually, a SMTP service provides 2 submission services:

• An implicitely secured one on *:465.
• An explicitely secured one (with STARTTLS) on *:587.

The user should use the first one but in the context of the non-existence of it, the second one is available. Usage and arguments are the same. However, default value of port is the default value of your operating system (see Unix.getprotobyname).