We have a fairly standard commercial e-mail setup, we use Outlook and Exchange and have a very well known third party service providing spam & phishing filtering and virus and malware scanning.
I ran a test using the Kaspersky version of the EICAR standard file (for those who don't know EICAR is a standard test used to trigger your anti virus response without any risk of virus infection - more here Kaspersky EICAR)
The EICAR com file is compressed in a zip file which I attached to a mail and sent it to several people, all users received the mail with the attachment in tact and with no warning messages to the user and no alerts on the mail scanner log, the file just bypassed the scanner. On opening the attachment Microsoft Intune immediately responded and cleaned the 'virus'. For comparison, i sent the same email to my Gmail address which filtered out the message.
Are there any valid reasons that a mail virus scanner would not detect the EICAR file as a real virus and respond accordingly?