When a new mailbox is added to Office 365, it has a default limit set for the size of emails it can send and receive. For emails it receives, this is around 36MB. For emails it sends out, it is around 35MB.
The limits are applied to emails after Office 365 has completed it’s transcoding, formatting the email based on the Office 365 platform. Because of this, other systems may show the email size differently.
The maximum available size for Office 365 is 150MB for inbound and outbound messages. It is still recommended to allow 33% headroom on an email for transcoding, so a limit of 150MB should apply a real usage limit of 112MB.
Change Email Size Restrictions
Note: If you don’t see the Admin Centers section, press Show More
Note: The limit is 153600KB (150MB) for both options.
Once changed, please allow up to 1 hour before attempting to send messages that exceed the previous limit.