Change Maximum Message Size limits in Exchange Online

Until recently the largest allowed e-mail message size in Office 365 was 25 MB as the public “softlimit”, and a 35 MB technical “hardlimit”.

Although most e-mail messages are much smaller than the 25 MB limit, attaching files to an e-mail will quickly get you over the message size limit, resulting in a Non-Delivery Report (NDR) and the message not getting to the recipients.

But Microsoft are now increasing the allowed maximum message size to 150 MB in Office 365, and provide Office 365 administrators with the opportunity to customize the maximum message size from 1 MB up to 150 MB.

It is important to note, that the default maximum message size for Office 365 mailboxes will remain at 25 MB, and Microsoft have announced that they have no plans to change the settings on existing accounts, as some organizations may not want to change them at all.

So Office 365 Administrator can now customize the maximum allowed message size limit for any or all of the organizations mailboxes. The limits can be changed in the Exchange Admin Center, but this requires Global Administrator role privileges.

To change the default maximum message size for your whole tenant, open Exchange Admin Center and select “Recipients“, then “Mailboxes“,  now click on the three dots “” and in the drop-down menu, click on “Set default message size restrictions“.

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In the new window, you can specify the maximum message size (up to 153600 kilobytes) for the mailboxes you create in the future – but not for the current mailboxes.

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To edit multiple mailboxes, select “Recipients“, then “Mailboxes” and select multiple mailboxes.
Look to the right in the “Bulk Edit” pane and locate the “Message Size Restrictions” section. Here you click on “Update” to open a windows where you can specify the limits for maximum message size in KiloBytes.

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The maximum allowed size is 153600 KB (150*1024) for both sent ad received messages. Click Save to commit the new settings.

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Finally, to set the allowed message size for a single mailbox:

Select “Recipients“, then “Mailboxes” and select a single user mailbox.
Click “Edit” (Pencil icon) and select “Mailbox Features“. Scroll down and locate the “Message Size Restrictions” section where you click “View details“. A window opens where you can modify the limits for maximum message size in KiloBytes.

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Click Ok to commit the changed settings.

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To update maximum message size on a large number of mailboxes (1000+), it may be more appropriate to use Remote PowerShell. Use the following commands:

Update a single mailbox Set-Mailbox -Identity -MaxSendSize 75MB -MaxReceiveSize 75MB
multiple mailboxes 
(“alias1”, “alias2″, “alias3″) | % {Set-Mailbox –Identity $_ -MaxSendSize 75MB -MaxReceiveSize 75MB}
Update all mailboxes  Get-Mailbox | Set-Mailbox -MaxSendSize 75MB -MaxReceiveSize 75MB
Update the default settings
(For mailboxes you create in the future.)
Get-MailboxPlan | Set-MailboxPlan -MaxSendSize 75MB -MaxReceiveSize 75MB

Please expect the Exchange Online service to take some time to process and replicate a large number of changes; expect roughly 5-10 minutes per 1000 mailboxes and additional 15-30 minutes for cross-service replications to complete –  before announcing that the new settings can be used 🙂



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