DETAILS

The virus scanner on the mail server is configured to reject email messages with zip files whose uncompressed content is more than 15MB.

This is a safety net for the virus scanning system, so that an abusive user cannot launch a denial of service attack on us by sending compressed files that contain large content. 

In order to scan files, the virus scanner must uncompress them. If the file content of a zip file is large, then this could lead to filling up the disk space of the server, which can degrade mail services.