One thing that we are considering is providing our staff with explicit instructions to: 1) Delete deleted items folder from Outlook; 2) Delete items in Sent folder from before a particular time period (i.e. 8 months); 3) Clean up sub-folders and folder heirarchies to remove old and unused subfolders and consolidate where possible, understanding that sub-folders will migrate to Front as personal tags.
I did not take these steps when I migrated, and am missing many emails as there is a $50k cap, and have had to do a lot of post-migration cleanup of personal tags.