Current default message size limit on Office 365 is configured at 36MB. The guide from Microsoft on how to increase these limits still points to the old Exchange Admin Center.
The current URL to make these changes is at https://admin.exchange.microsoft.com/
- Login with your admin credentials, select mailboxes.
- Select the user mailbox to increase message size.
- Click Mailflow setting and select Message size restriction.
- Change the value to your required size in KB. (100MB = 102400KB)
To make changes for all mailboxes, select the checkbox besides Display name and follow the same steps listed above.
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