Bouncing is evil

Below two typical mail setups

Origin Mail User Agent sends via origin Mail Transport Agent to destination;

 +-----+    +-----+             +-----+    +-----+
 | MUA +--->+ MTA +---> ... >---+ MTA +--->+ MUA |
 +-----+    +-----+             +-----+    +-----+

Origin Mail User Agent sends directly to Destination MTA;

 +-----+             +-----+    +-----+
 | MUA +---> ... >---+ MTA +--->+ MUA |
 +-----+             +-----+    +-----+

The only bounces to occur should be bounces generated by the origin MTA; This is the ONLY type of bounce which is acceptable. All OTHER bounces should be avoided!

 +-----+    +-----+
 | MUA +--->+ MTA +-+
 +-----+    +-----+ |
   /|\              |

The remote MTA should NEVER ever send any bounce messages ! ! !

 +-----+    +-----+             +-----+
 | MUA +--->+ MTA +---> ... >---+ MTA +-+
 +-----+    +-----+             +-----+ |
              /|\                       |
               +------< ... <-----------+

This of course also means that you should not use a smarthost or fallback MX(es). This also means that the origin MTA should only accept envelope addresses which are local domains. Otherwise it could send bounce messages to non local domains.
If you want people to reply to a domain other then the local MTA local domain(s), specify a reply-to address. The reply-to will be used by the remote MUA, the envelope from by the local MTA.

If you have a permanent internet connection it's probably a good idea to run your own mailserver. If you do, do not use your ISP's mailserver. Don't use them as a smarthost and don't use them as a fallback MX. If you do use their mailserver, let them do all the spam filtering. Don't do any filtering of your own. If you are not satisfied with your ISP's mail filtering, set up your own server and your own filtering. If your ISP blocks port 25 (either incoming, outgoing or both) change ISP.

Any other setup will spread viruses and joe-jobs and amounts to harassment.

Be careful which MTA you select for the job. Exim 3 for instance, won't do. Exim 4 + exiscan patch on the other hand, does do the job.
Postfix is also OK.
Qmail sux ! It's based on fundamentaly broken concepts. Don't use Qmail! (according to some, some versions of Qmail do reject).