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If you have right to access the ISP router, then I bet you should try to enable level 7 logging on the router interface to see whether the traffic able to reach both.
I have a Pix 525 unit with the mailguard enabled and running without any problem with the attachement.. I think the best you can do is to simulate the environment that bypass the pix firewall.. for example to use outlook express client to send a emai...
