Change Email Size Restrictions in Office 365


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

  1. Log into the Microsoft 365 Admin Center using global admin credentials for the tenant: https://admin.microsoft.com
  2. Under Admin Centers, select Exchange

Note: If you don’t see the Admin Centers section, press Show More

  1. Go to ‘Recipients > Mailboxes’

  2. Double click on the mailbox you want to edit

  1. Select Mailbox features

  1. Under Message Size Restrictions, select View Details

  1. Enter the new limit (in Kilobytes (KB)) for both Sent & Received messages

Note: The limit is 153600KB (150MB) for both options.

  1. Press OK

  2. Press Save

Once changed, please allow up to 1 hour before attempting to send messages that exceed the previous limit.