Thanks Graham, We just started migrating about a few weeks ago. The major part of the migration is in effect now so I think you have a point about it being halfway through the migration. The error has not popped up in a few days so I think you are correct but still monitoring the issue.
I'm not on the messaging team, just on the Mac imaging team so I my guess is built-in accounts.
I'll update you on what I find out.
Thanks again.
James