Whenever administrators migrate mailboxes to Exchange Online, an onboarding limit in terms of message size always apply in order to avoid migrating huge messages and potentially impact service. Up until now this limit was 25MB (attachment size) plus overhead, so approximately 36MB in total. If a mailbox contained any emails over this size, they would simply be skipped and administrators had to find alternative ways to import them. To be honest, not many companied allow such big emails, but I have worked with a few that do, which would cause an issue if migrating to Office 365...
Now, this limit has been increased to 150MB (and is already available to all customers)! However, please have the following in mind:
This new limit only applies to onboarding moves using the native Mailbox Replication Service (MRS) and which target AD user objects with a RecipientType of MailUser (MEU) that do not have MaxReceiveSize stamped. For example, hybrid deployments;
All other data migration solutions, both using Microsoft tools and 3rd party tools target UserMailbox (MBX) AD users (i.e., rely on the mailbox already being created in Exchange Online). The increased onboarding message size limit does not apply to these solutions which will use the limit (MaxReceiveSize) configured on these MBX objects. The maximum-allowed onboarding message size limit for these solutions remains 36MB. This includes merges (staged and cutover migration), IMAP, EWS, MAPI, 3rd party tools, and PST Import.
The good news is that soon administrators will be able to customize the send/receive size limit, enabling them to modify the MaxSendSize and MaxReceiveSize limits on their MBXs and MEUs, effectively overcoming this limitations (hopefully).
The Exchange Online Service Description has now been updated to reflect this change.