Unfortunately I don't use Outlook nor Office 365 for my e-mail so I can't test it for myself.
According to Google, Outlook / Microsoft 365 combination may use MAPI for communication instead of IMAP/POP3/SMTP.
And I'm not sure Mail shield supports MAPI in this case.
If you can see e-mail header sent/received from/to your client, it should have strings like these when scanned.
X-Antivirus: Avast (VPS 201207-0, 2020/12/07), Outbound message
X-Antivirus-Status: Clean
Anyway, we may need devs or members familiar with these experiences.