The ctmail.com domain does belong to Thirtyseven4 Antivirus. The domain itself actually belongs to CommTouch, the Cloud-based engine we integrate as part of our Browser Protection module technology (same engine used by Microsoft, Google, Mcafee, etc).
The process is this...when a user opens a webpage, the Browsing Protection module quickly (1/20th of second) checks with ctmail.com and verifies if that it is a clean domain. If clean, our module will store that domain as safe so that this domain will not get checked again.
Using the Browseing Protection module, we can actively protect a school district from the malicious sites themselves instead of relying on file detection.
We do suggest that schools free the ctmail.com traffic or white list it and its sub-domains from their Content Filtering software.