Linux mail server just for forwarding mail
From experience:
- qmail for speed and security
- exim for configurability
- postfix for simplicity
- sendmail for masochists
Your questione is too vague to give a recommendation for what you're doing. The most important consideration is likely to be where you get the config for each forwarding email address.
Personally, I prefer Postfix. Easy to setup, easy to maintain. It's flexible enough to handle what you are trying to do.