As John said, the databytes configuration should do what you are asking. It'll reject the message right away if the sender uses the extension that specifies the size in MAIL FROM and if not then it'll stop buffering the data after the configured number of bytes has been received (and then reject the mail when the sender is done).
I see your wisdom -- I've implemented this and it works great. Thanks for making QP smart :)
-Jared -- Inbound and outbound email scanned for spam and viruses by the DoubleCheck Email Manager: http://www.doublecheckemail.com
