Outlook - Email Limitations
Blocked File Types
For security purposes certain file types are restricted from being sent or received by email. You may encounter an error if you attempt to send these file types, or your recipient may not receive the attachment.
The following file types are blocked by DMU explicitly.
'vbs' or 'sct' or 'scr' or 'reg' or 'pif' or 'pcd' or 'msi' or 'ms' or 'mdb' or 'lnk' or 'jse' or 'isp' or 'ins' or 'inf' or 'hta' or 'hlp' or 'exe' or 'crt' or 'cpl' or 'com' or 'cmd' or 'chm' or 'bat' or 'zip'
Additionally, Microsoft may automatically block certain file types that have a high likelihood of having a malicious payload.
You can find more info on that here.
Attachment File Size Limits
Due to storage and bandwidth limitations, attachment file size in outlook is limited. The Limit has been updated as of 5/16/25 to 100MB.
If you need to send a file larger than 100MB please contact the Solutions Hub for assistance with the best options for sharing large files.
With this increase to 100MB, sending a large attachment to external recipients could result in a rejection or bounce message if their organization's attachment limit is less than the size of the sent file.
External Forwarding
Forwarding of email to an external mailbox is not allowed from any DMU mailbox.
Sending Secure and Confidential Information
Confidential information detection is turned on for all mailboxes. You may receive a warning if you attempt to send unencrypted confidential information.