File Attachment Limits and Restrictions in Outlook

Outlook blocks certain file types and has a file size limit of 150 Megabytes when sending to another Office 365 mailbox, and 25 Megabytes when sending to non-Office 365 mailboxes.

Blocked file types

View the Blocked attachments in Outlook page for a list of blocked file types and tips for saving files as acceptable file types. 

Note: Microsoft recently banned additional file types, including Java, Python and PowerShell extensions. View the full list of recently banned file types. 

If you try to send a blocked file type from Outlook, you will receive delivery failure message from the system. If someone tries to send you a blocked file type outside of Microsoft 365, their system may not block the message; however, Microsoft 365 will block the message and you won't receive their file. 

If you need to exchange a file type blocked by Outlook, use LiquidFiles.

File size restrictions

As a file is attached to a message, its size can inflate beyond the size of the attachment limit due to encoding. It is also possible that the receiving party, if outside of the Microsoft 365 system, may have a lower message size limit that will not allow larger attachments to make it through. 

If what you need to send appears to be too large, try zipping the file to compress its size or check out our selection of file sharing tools.