I recently move 200 accounts (700GB) customer to my ME Professional y tried via migration tool but it takes really long time. Then I use the local OST (create pst file) to import messages.
Now after 3 weeks I have around 20 accounts never completed the migration in any way.
The customer has delays up to 2 hours sending or receiving email in random accounts some cases, emails appears only webmail but no Outlook or mobile.
I tried to reindex several times and i tried deleting inde.xml, decrease time for sync in outlook, change dns, more bandwidth,