End users can send large attachments if your mail server or secure email gateway (SEG) supports a higher maximum message size.

If your organization uses a mail server other than Microsoft Exchange or an SEG, refer to the documentation of that mail server or SEG.

Microsoft Exchange Online

The default maximum message size in Microsoft Exchange Online is 25 MB. You can increase this limit up to 150 MB. This means that the message and its attachments together cannot exceed 150 MB. If the limit is exceeded when the user adds an attachment, Outlook shows a warning.

Follow these steps to increase the maximum message size in Microsoft Exchange Online:

1. Go to the Exchange Admin Center.
2. Click Recipients in the menu on the left.
3. Click Mailboxes.
4. Click Mailflow setting.
5. Click Message size restriction.
6. Increase the current value shown for Sent messages maximum size(KB).
7. Click Save.

The maximum message size is increased.

Microsoft Exchange On-Premise

The default maximum message size in Microsoft Exchange On-Premise is 10 MB. This limit can be increased to 25 MB, 100 MB, or set to unlimited. If the limit is exceeded when adding an attachment, Outlook shows a warning.

Most organizations set the limit to 25 MB. This means that the message and its attachments together may not exceed 25 MB. An unlimited maximum message size may cause delivery problems, since not all receiving mail servers can handle it.

Follow these steps to increase the maximum message size in Microsoft Exchange On-Premise:

1. Go to the Exchange Admin Center.
2. Click Mail flow in the menu on the left.
3. Click Send connectors.
4. Click more_horiz.
5. Click Organization transport settings.
6. Click limits.
7. Increase the current value shown for Maximum send message size (MB).
8. Click Save.

The maximum message size is increased.