The latest Office 365 “What’s New” post introduced a change to the onboarding message size limits. The old limits of 25 MB have been increased to 150 MB while onboarding mailboxes to Office 365. This is great news and I know many customers will be very happy with this. Well, I know even more consultants that will be sleeping better at night with this increased flexibility!
The verbiage of the official announcement stated that:
Office 365 Exchange Online message size onboarding limit increase — We are making a change to allow customers to migrate larger mail messages to Exchange Online. We now allow messages up to 150MB to be migrated to the service. The change is available immediately to all customers and is published as a new limit in the Exchange Online limits page in the Office 365 service description. We are not changing other limits such as mailbox size or maximum send/receive limit for messages. This change enables customers with large messages to easily migrate their existing content to the message.
The service description reflecting the change has been updated as promised here. After the initial excitement of the announcement wore off many in the community, myself included, started to have questions around how this new change was being implemented. For instance, would the traditional third-party migration methods be allowed to take advantage of this increase? Some of these tools leverage EWS or IMAP to migrate data into O365. After some internal discussion with Microsoft they have clarified how the change will be implemented.
The following two statements are true:
1 The 150 MB onboarding change is only valid for native MRS-based moves and not for EWS, MAPI and IMAP.
2 Any migration methods that rely on the mailbox already being created in O365 will be subject to the 25 MB limit.
The changes to the onboarding message size will only apply to moves that utilize MRS and users that are staged with an MEU RecipientType in hybrid configurations. Other third-party solutions that target UserMailbox will be subject to the traditional message limit, as MRS is not being called.
Also, some have asked if any changes need to be made to the send connectors for hybrid mailbox moves. Since mailbox moves are a function of EWS via MRSProxy, they are not related to transport. The transport rules will not kick in because a mailbox needs to complete the migration before the mailbox plan is associated with it. Once you have a mailbox plan associated with a mailbox the transport limits will then be enforced.
Hopefully this helps to clarify the onboarding message size changes. Don't mind me as I start to smile while moving large attachments into O365!