libESMTP is a library to manage posting (or submission of) electronic
mail using SMTP to a preconfigured Mail Transport Agent (MTA) such as
Exim. It may be used as part of a Mail User Agent (MUA) or another
program that must be able to post electronic mail but where mail
functionality is not the program's primary purpose. libESMTP is not
intended to be used as part of a program that implements a Mail
Transport Agent.

libESMTP is an attempt to provide a robust implementation of the SMTP
protocol for use with mail clients. It is being developed as a reaction
to the experience of incomplete or buggy implementations of SMTP and
also to help remove the need for the installation of MTAs on
workstations which only need them to provide a sendmail command for a
mail client to post its mail.

WWW: http://www.stafford.uklinux.net/libesmtp/