A few from a quick search on Freshmeat.net ..
archivemail:
http://freshmeat.net/projects/archivemail/
archmbox (archives mbox style mailboxes):
http://freshmeat.net/projects/archmbox/
Lurker (looks pretty sweet, actually.. might check into this for my own uses lol):
http://freshmeat.net/projects/lurker/
mbox2mysql (not a text file, but archives mbox format E-mail to mysql):
http://freshmeat.net/projects/mbox2mysql/
Mirphealy Mail Archiver (this one is in PHP):
http://freshmeat.net/projects/mirphealy/
Probably not exactly what you need... but a place to start. If you are looking for a ticket tracking system (which you said you aren't, but I looked at the thread to see what people were recommending)... the few we use are;
PerlDesk (decent, a few problems though but easy to work around):
http://www.perldesk.com/
RequestTracker (pain in the ass to set up, but very very nice):
http://www.bestpractical.com/rt/
The nice thing about request tracker is that there are a lot of 3rd party reporting modules, etc written for it... 99.9% of them are free
Would be pretty easy to write a spam reporting module.
If you are truely just looking for something which will dump all E-mail sent to a set address to a text file, any half-way competant coder should be able to do something in perl in an hour or less.
CGI