smtp.pl -- Send E-mail through SMTP

This module provides a simple means to send E-mail from a Prolog application. Here is a simple example:

send_message(Out) :-
        format(Out, 'Hi Alice,\n\n', []),
        format(Out, 'Want to go out tonight?\n\n', []),
        format(Out, '\tCheers, Bob\n', []).


?- smtp_send_mail('alice@wonderland.com',
                  send_message,
                  [ subject('Tonight'),
                    from('bob@wonderland.com')
                  ]).

This library currently supports good old  SMTP, encrypted and authorized
ESMTP. Both SSL/TLS and STARTTLS  encryption is supported. Authorization
is supported using =PLAIN= and =LOGIN= methods.

Data is currently being sent using the =DATA= keyword.

@tbd    Support more advanced data transport extensions such as sending
        MIME messages.

 smtp_send_mail(+To, :Goal, +Options)

Send mail using SMTP. To is the e-mail address of the receiver. Options:

• smtp(+Host) the name or ip address for smtp host, eg. swi-prolog.org
• from(+FromAddress) atomic identifies sender address. Provides the default for header(from(From)).
• date(+Date) Set the date header. Default is to use the current time.
• subject(+Subject) atomic: text for 'Subject:' email header
• auth(User-Password) authentication credentials, as atoms or strings.
• auth_method(+PlainOrLoginOrNone) type of authentication. Default is default, alternatives are plain and login
• security(Security) one of: none, ssl, tls, starttls
• content_type(+ContentType) sets Content-Type header
• mailed_by(By) add X-Mailer: SWI-Prolog <version>, pack(smtp) to header iff By == true
• header(Name(Val)) add HName: Val to headers. HName is Name if Name's first letter is a capital, and it is Name after capitalising its first letter otherwise. For instance header(from('My name, me@server.org')) adds header "From: My name, my@server.org" and header('FOO'(bar)) adds "FOO: bar"

Defaults are provided by settings associated to this module.

Listens to debug(smtp) which for instance reports failure to connect, (computation fails as per non-debug execution).

Arguments:

To	- is an atom holding the target address
Goal	- is called as call(Goal, Stream) and must provide the body of the message.