"POP before SMTP" is a not standarized change in the SMTP protocol. It
is used in order to protect the SMTP servers from spammers. If you send
an e-mail, you have first to log on to the POP server and then you can
send the mail via SMTP. The advantage is the spammer protection but on
the other side, lots of mail clients do not support "POP before SMTP"
an produce errors instead of sending the e-mail. But there is a really
simple solution for the client problem. Take a look at
It works with all Windows E-Mail clients and it is for free.