spamassassin keyword blocker for outgoing mail?
 

my application allows site visitors to ask my registered users questions by email. the email is sent by my server. What i'd like to avoid is having my registered user's get emails from my email address with foul language, obscenities, porn keywords, ...

I was thinking of doing keyword filtering of the message body in my application but I then heard about spamassassin. I am using postfix as my mail server. can spamassassin do the job for me?

yes. after integrate spamassassin with postfix mailserver. However I use sendmail.

Look at AMavisD as well if you want combined Virus Scan and SpamAssassin. If not SA will work great with PostFix. There are plenty of how tos around. So many it will make your head swim and they are all a bit different.