Please find a list of migration issues and resolutions that I captured during my last customers migration:-
Flood mitigation on Threat Management Gateway (TMG) - http://community.office365.com/en-us/w/exchange/office-365-move-mailbox-fails-with-transient-exception.aspx
A large item was encountered: Item (IPM.Note) Subject:"4600562666", Size: 42.07 MB (44,113,161 bytes), Folder:"Inbox
A large item was encountered: Item (IPM.Note) Subject:"Here's My Card", Size: 42.43 MB (44,493,004 bytes), Folder:"Inbox"
A large item was encountered: Item (IPM.Note) Subject:"", Size: 48.49 MB (50,848,177 bytes), Folder:"PRIVATE EMAILS"
ExMerge - http://support.microsoft.com/kb/328202
Search-Mailbox – Needs to be a 2010 mailbox
To be able to use the -DeleteContent Parameter you need to have "Mailbox Import Export" permissions. Without these permissions, the parameter -DeleteContent is not available for the Search-Mailbox Command.
1. Create a new Security Group
2. Enter the following command in Exchange Management Shell (replace the Security Group name accordingly):
Warning: Failed to clean up the source mailbox after the move.
Error details: MapiExceptionNotFound: Unable to delete mailbox. (hr=0x8004010f, ec=-2147221233)
The issue occurs when migrated accounts cannot clear homemdb, homemta, msexchhomeservername and msexchguid attributes from the source server and set the TargetAddress on the source side which it is supposed to do after mailbox move completes (flipping of the attributes starts after 95% completion of the move). A script can be created or apply this hotfix - http://support.microsoft.com/kb/940012
Pretty much the same issue as large item count
Written by Daniel Kenyon-Smith