Here's the thing. I need a daemon running on the server which basically will open an smtp connection, then it will expect calls to the process to send e-mails through the opened connection.
This is to avoid connecting to the smtp every time, instead I can receive multiple calls to the process but connection only one time.
By the way, the SMTP always authenticates with the same user and password, no need of a different authentication by each user.
Does this make any sense?
You could use a php mailing class that connects only once to the smtp server and uses this connection for all the emails that are sent during the scripts execution. If I'm not mistaken pear php will do that.
I think you should seriously consider using an existing MTA which can do authentication and queuing, then just send the messages directly to there and let it relay.
Use (for example), sendmail. This is not a recommendation just an example. Make your own choice.
Making a custom server to do all of this is fraught with problems and will definitely have interoperability problems (because SMTP is such a bag of crap)