End users can send large attachments that your mail server or secure email gateway SEG processes if you can increase the message size limit in Microsoft Exchange.

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

Microsoft Exchange Online

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

Do these steps to increase the message size limit in Microsoft Exchange Online:

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

The maximum message size is increased.