My personal preference is Postfix (for lesser patching ;-)), but choose the MTA you know/like best (or which you get best support in your environment) be it exim, postfix, qmail, or even sendmail (courier-mta i would not recommmend ...). I would prefer handing the Mail over to a full-blown MTA, if available (with filtering mechanisms like spamassassin attached etc.)

There are spam sources enough out there, so be sure to validate your code ...
Just my 2 ct.
hth MH