Sending large emails from Office 365- Admin Help (Upto- 150 MB)

Microsoft 365 allows administrators to configure the maximum email message size for sending and receiving. While the default limit is 25 MB, it can be increased up to 150 MB per mailbox.

Key Points

• Default Limit: 25 MB for both sending and receiving.
• Maximum Configurable Limit: 150 MB.
• Applies To: Individual mailboxes, shared mailboxes, and the organization-wide transport settings.
• Encoding Overhead: Due to Base64 encoding, the actual attachment size should be smaller than the configured limit (e.g., a 100 MB file may result in a ~133 MB message).

How to Configure Message Size Limits

1. For Existing Mailboxes

1. Go to the Exchange Admin Center: https://admin.exchange.microsoft.com
2. Navigate to Recipients > Mailboxes.
3. Select the mailbox you want to modify.
4. Click Mail flow settings.
5. Under Message size restrictions, click Edit.
6. Set the Maximum send size and Maximum receive size (up to 150 MB).
7. Click Save.

2. For New Mailboxes (Default Setting)

To apply a default message size limit for all future mailboxes:

• Go to Mail flow > Rules or Mail flow > Settings in the Exchange Admin Center.
• Adjust the default message size restriction under Organization settings or Transport settings.
• This ensures that all newly created mailboxes inherit the updated size limits.

Notes

• These settings only apply to Exchange Online mailboxes.
• If you're using Outlook clients, ensure they are updated, as older versions may not support sending large messages even if the server allows it.
• OneDrive or SharePoint is still recommended for sharing very large files.