Microsoft 365 - How To
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      Change Mailbox Message Size Limits

      You can increase the size limit in which your users can send messages. Keep in mind that most mail servers don't accept messages larger than 35 MB.

      1. Go to admin.microsoft.com and select the Exchange Admin Center on the left side.
      2. Click Mailboxes under Recipients.
      3. Double click the user that you want to edit and click mailbox features.
      4. Scroll down to Message Size Restrictions and click View details.

      5. Change the number in KB and then click OK.
      6. Click Save.