Recently we upgraded our ASA to software version 7.2(2) and all of a sudden I started to get compliants from users that they were unable to receive email from Gmail with attachments.
I decided to validate this claim by trying to send an email with an attachment from Gmail to myself and I it seemed to hold up as I never received the mail but two days later Gmail sent me a "Delivery Status Notification (Delay)" saying the message could not be delivered because of the following reasons:
Technical details of temporary failure:
Since my postfix server (behind my firewall) was not even logging a connection attempt from Gmail, I suspected it had to be something with the stateful inspection of ESTMP on the ASA. I removed stateful inspection of ESMTP from the global ploicy map:
ASA(config)# policy-map global_policy
and walla I was now able to recieve email with attachments from Gmail again.