Hi depam,
Some time ago, some clients could not work perfectly with POP before SMTP (when you did click in "send and receive" instead they authenticate in pop before send the messages, they tied to send the messages first). So since then, I've used SMTP auth and I don't know the POP before SMTP status today.
I've used postfix for years without problems.
I also worked with qmail, but I think that postfix easier to setup and administrate (my personal opinion).
You can easily integrate spamassassin and/or an anti-virus with amavis (
http://www.ijs.si/software/amavisd/)