I recommend and install Calyptix’s Access Enforcer all-in-one network security appliance at many of my customer sites. Compared to my experience working with some other security appliances, I find the Access Enforcer very easy to install and maintain.
If you work with Calyptix, you may not be aware that they do “mail bagging” automatically if you enable SMTP filtering. “Mail bagging” simply means that if (for whatever reason) incoming mail cannot be delivered to your on-premise mail server, the Access Enforcer will hold (bag) it. Once your mail server is back online, it will release the emails.
The Calyptix KB article on mail bagging was updated today to address the proper requirements for using this feature.