MailScanner contains a flaw that may allow a malicious user to bypass virus scanning of ZIP files. It is possible that the flaw may allow ZIP files containing viruses to bypass scanning. The issue is due to the program not correctly parsing size checks on the contents of zip files vs the zip files themselves.
Upgrade to version 4.42.2 or higher, as it has been reported to fix this vulnerability. An upgrade is required as there are no known workarounds.