Exchange Online Protection scans all email attachments for executable files and blocks them to prevent the spread of malware. If executable files are detected, the entire message will be rejected and the sender will receive a nondelivery report with the following comment: "This message was blocked because its content presents a potential security issue."
File extensions blocked include:
- .ade
- .adp
- .ani
- .bas
- .bat
- .chm
- .cmd
- .com
- .cpl
- .crt
- .hlp
- .ht
- .hta
- .inf
- .ins
- .isp
- .job
- .js
- .jse
- .lnk
- .mda
- .mdb
- .mde
- .mdz
- .msc
- .msi
- .msp
- .mst
- .pcd
- .reg
- .scr
- .sct
- .shs
- .url
- .vb
- .vbe
- .vbs
- .wsc
- .wsf
- .wsh
Zip files (.zip) containing executable files will also be blocked, unless they are password protected or encrypted. Never open any email attachments from unknown senders.