router problem with spam

I've been having a weird problem with a dsl router dropping communication during pop3 transfer of certain spam emails. I'd appreciate any insight to this problem.

My configuration:
* Linksys 8 port Dsl/Cable Router (BEFSR81 v3) NAT enabled
* 100baseT LAN with 3 Windows PC's and 1 Win2k Mercury mail server.
* DSL service to the router/Fujitsu modem.
* latest O/S updates, and router firmware are applied

The Problem:
Once in awhile a non-RFC compliant "bad" spam email is received by the mail server, and when the pop3 client attempts to read the mail, it will get "stuck" on the "bad" email and drop the connection, ending in a timeout error on the client side. (Problem was traced to the router, please bear with me...). The client can be external to the LAN or on the LAN, same problem occurs.

[I]Troubleshooting history:I]
1. Once the "bad" email is identified and deleted from the user's account, all email passes without a problem.

2. By process of elimination, the router was determined to be the source of the problem. First I took out the "bad" emails and put them on another mail server on a different network, and they passed perfectly (no DSL Router). I used the same mail server, same O/S. I also changed the NIC and cable out from the mail server. This basically just left the router as being the culprit. Then I updated the router firmware to the latest version, which cleared up problems with all the test cases I had.

I thought the problem was solved but now I have nother "bad" spam email that won't pass through.

My thought is that there is some sort of timing, or MTU problem that is a corner case with this particular router. Only these non-RFC compliant spam emails cause it to fail. Either a packet is not filled, or something I have no idea about is happening.

Any thoughts on troubleshooting the router or any ideas would be greatly appreciated.

thank you for all that read and probably were scratching your heads wondering

I ran some more tests and determined that the problem was still fixed by upgrading the router firmware as originally done to solve this issue. This was good news, because I had determined the reason for the firmware upgrade was a MTU problem that was fixed in the upgrade. All my test cases had passed after this fix.

The problem in this particular case turned out to be Norton Antivirus incoming email scanning was choking on the "bad" spam. This particular spam mail is blank and is missing lots of header information, such as the subject. It's a blank email. Once I disabled the incoming email virus scan, it worked fine.

sorry if this is a bit out of scope for this forum, but I thought it still was the router....maybe it will help someone.