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 an 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.

Microsoft Exchange On-premise

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

Most organizations will set the limit to 25 MB. This means that the message and any additional attachments together may not be bigger than 25 MB. An unlimited message size limit might cause delivery problems, as not all receiving mail servers might be able to handle an unlimited message size limit.

Follow the instructions below to increase the maximum message size in Microsoft Exchange On-premise:

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

The message size limit is increased.

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