The solution to "Abort Message" issues is most of the times not related to SMG/SBG directly, the best way to really troubleshoot this issue is to contact Technical Support so we can get a tcpdump when the issue occurs; however, in the most cases we were able to resolve this issues by:
1) Making sure there are no thirdy party proxies in between SMG/SBG, a common issue is Cisco ASA with ESMTP enabled (which is on by default) or Cisco PIX's fixup;
2) Timeouts during the data verb, you might try to increase the Inbound/Outbound timeout (depending where the issue is happening) up to 5 minutes (default is 30 seconds) to see if that happens.
If everything fails, like I said, I would contact technical support to further troubleshoot.