tye,
They (email addresses with periods immediately preceding the @) were also very common by Microsoft Exchange back when I was working at the DoJ. I am not sure if M$ has become more compliant. I am going to be updating this node with a variety of other ways at attempting to exploit open relays and I will /msg you when complete.

Update: Rather than enumerate them myself, go to http://www.abuse.net/relay.html and test an MTA you believe to be secure. It shows you all the email addresses it uses to test with (from and to). I also realized I had the relay syntax wrong. It is foo%bar.com@example.com. I have updated the prior node.

Update: I haven't provided a complete list of "rules" that I think such a theoretical module should include but having "John Smith"@example.com is another one that should be flexible. If I come up with more I will add them here but it has been a long time since I thought about such things. Oh, and I used to have to worry about non-SMTP addresses too like CC:Mail and GroupWise (fortunately not UUCP).