Microsoft will transition your existing Exchange Hosted Archiving data to the advanced archiving of Office 365; however, there are certain data types described below that will not be migrated to the new service. Customers will receive a full copy of their Exchange Hosted Archive (EHA) data on encrypted media once migration has completed.  The following messages are not supported in Office 365 and therefore will not be migrated:   

Messages greater than 25MB will not be migrated. Office 365 has a service wide limit of 25MB per message.

    Messages to un-provisioned Users will not be migrated. Before your migration begins all users for whom data needs to be moved must be provisioned in EHA. For more information on how to provision users in EHA, please see the EHA transition to Office 365 Prerequisites wiki-post on found here

Bloomberg E-mail and Bloomberg Instant Messages will not be migrated. Office 365 does not natively support the storage of Bloomberg messaging data.

Instant Messages will not be migrated. Office 365 natively supports archiving Microsoft Lync Instant Messaging. However Office 365 does not support the Instant Message (IM) formats that were supported by EHA:  MessageLabs EIM (previously Omnipod), Parlano MindAlign, FaceTime, Symantec IM Manager (IM Logic), and Akonix. 

Messages for disabled or deleted domains will not be migrated. E-mail in EHA that is associated with domains that have been disabled or deleted from the EHA Admin Center will not will not be migrated.