That makes things a lot more clear. It sounds to me like you've either got spammers, a specific kind of DOS attack (see the link below), or a fuzzy router somewhere.

In all likely-hood, it is just spammers trying to use your mail server. If you can figure out that its all coming from the same place, you could configure the router to just block that IP Address/Range.

Addresses about spam/relays:
http://www.ccs.neu.edu/howto/howto-relaying.html
http://www.hermes.net.au/web/relaying.htm

It could also be a relay authorization attack, a weird version of DOS:
http://www.vamsoft.com/orf/authattack.asp

If you have anymore questions, don't hesitate to ask.