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I have Exchange 2010 and Office 365 set up in full hybrid mode. I have migrated some mailboxes over and mail flows as expected. Some of the mailboxes had a non default Receiving Message Size set while they were still on premises. Now that the mailboxes have been migrated this old limit is overriding any limit set in Exchange Online.

When looking at the Remote User Mailbox in the Exchange 2010 interface I can see the Maximum Message Size under the Mail flow Settings tab but it is greyed out and hidden behind a padlock indicating I don't have the permissions to modify it.

How do I go about changing these limits so that the migrated mailboxes respect the Exchange Online size limits?

2 Answers 2

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Because the mailbox was migrated to Office 365 you have to set this in Office 365/Exchange Online:

  1. Connect to Exchange Online

  2. Run: Get-Mailbox -Identity mailbox | Set-Mailbox -MaxReceiveSize xMB -MaxSendSize xMB

where mailbox is the mailbox you want to configure and where xMB is the size you want to set.

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As joeqwerty said, because you migrated your mailbox to exchange online, you need to change these limits in exchange online. The command joeqwerty provided seems to be unavailable in exchange online, you need to use the following command to make changes to the mailbox.

Set-MailboxPlan mailbox -MaxSendSize xxMB -MaxReceiveSize xxMB

For more details: Configuring Max Email Message Size Limits for Office 365

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  • Both yours and joeqwerty's commands work to change the mailbox size limits for the Exchange Online mailbox. Its the on premises Remote User Mailbox object in Exchange 2010 that has this setting and is greyed out. Our MX records still point to the on premise server as 90% of our mailboxes haven't been migrated yet so this receive setting overrides any setting on the online mailbox. Feb 12, 2020 at 8:19
  • It sounds like maybe you're dealing with a different issue. You can't set the limit on the on premises remote user because the mailbox is in Office 365 and needs to be set there. What you might dealing with is the message size limits of the on premises to Office 365 connector. Are you running into issues sending large messages between on premises and Office 365 mailboxes? If so, then the connector limits are probably the issue. IIRC, the connector limits are set to 10MB.
    – joeqwerty
    Feb 19, 2020 at 12:37

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