One went into BPOS (one of the dumbest acronyms EVER!) which is in reality Exchange 2007, which does NOT have submission quotas. (So now you know where this is heading)
The other site went into Live @ Edu which, despite their having had their submission quotas removed for purposes of migration, found the quotas very much in place.
So this is about what you'll see if one of your users hits submission quota in a migration.
In this example we kept inserting meetings until we hit quota. So one went in fine (you see the guest list and responses) and one did NOT (it says "Invitations haven't been sent for this meeting").
Once your submission quota rolls over (and when is that exactly?) you can send this and it'll go out like a regular meeting invitation. Not fatal in a migration, but we agree, it is darned annoying.