Included in the latest round of Office 365 announcements is details of the new Exchange Online message size limit for onboarding (mailbox migrations). Previously the limit was 25mb (attachment size) + overheads, which roughly came out to 35mb total message size.
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.
I confirmed with Microsoft that this also applies to offboarding mailbox moves as well, which is good news for customers who have had to skip (and therefore lose) oversized messages when offboarding a mailbox.
You may be wondering how a message that exceeds the previous service limits could possibly exist in an Exchange Online mailbox. That is indeed a good question.
Anyway, enjoy your new message size limits when migrating mailboxes in and out of Office 365 
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