One of our customer seems to have a very old email system, those that ask you to authenticate to the POP3 server before allowing you to send messages through the SMTP server. Regrettably, we have to believe in what our customer tell us for we cannot access their facilities. But as far as I remember, years ago there were mail systems that once you log into the POP3, the STMP server is kept open for a few minutes for the client IP. Our application sends messages by using System.Net.Mail.SmtpClient which seems to be unable to authenticate to those kinds of servers. Is that correct? If so, what would be the simplest workaround? I was thinking of a minimal POP3 implementation (just the login part of the protocol). Would that work? Thanks in advance.