Few months back, the Exchange Online team announced a long awaited feature – increasing the maximum size for messages to 150MB. This was actually a two-part solution, with the first part being allowing migration of messages larger than 25MB (35MB) during remove mailbox moves via the Mailbox Replication Service. This made a lot of people happy, but unfortunately didn’t apply to all scenarios, as explained in the service description document:
The size of messages to be moved into Exchange Online are computed by Exchange Online. Versions of Exchange prior to Exchange Server 2013 may report a smaller item size. This limit applies to move based migrations using any supported Exchange Mailbox Replication Service. Other migration methods (Cutover, Staged, IMAP, PST) and other third party tools are limited by the general message size limit.
Now, finally, the anticipated second part of the change has begun rolling out to Exchange Online tenants. While the actual blog post doesn’t mention anything about onboarding/migration and only talks about increasing the maximum send/receive size, the mechanics behind this change mean that it will apply to migration as well. Namely, Cutover, Staged, PST Import/Export, IMAP, EWS based methods and tools will all be able to take advantage of the new methods. Depending on the method used (meaning does the method provision the mailbox or uses an already existing mailbox), you will have to use one of two options:
1) Increase the max send/receive size for mailbox(es) via:
Set-Mailbox vasil -MaxSendSize 150MB -MaxReceiveSize 150MB
2) Increase the default settings for new mailboxes via:
Get-MailboxPlan | Set-MailboxPlan -MaxSendSize 150MB -MaxReceiveSize 150MB
Note that there might still be some issues with migrating large messages because of method/protocol specifics. For example, the maximum message size for EWS is controlled by different settings, which seem to not have been affected by this change. Instead, the default limits for Exchange server are used (which means max size around 48MB/64MB as explained here). Hopefully, the team will soon post some more detailed guidance. But overall, a lot of people will be happy with the new limits!