Indexes of the imported mail folders should be automatically rebuilded so there wouldn't be a need to go manually into every folder (could be tens of them) or do some searches (+ mark them as read (Bug 219269)). This would greatly help with the migration process.
Related but more general - Bug 81141
Didn't this get fixed very recently?
No, we fixed a regression that made clicking on the folders not rebuild the index, or something like that. What this is suggesting is building the indexes automatically as part of the import process. It wouldn't actually be very hard...but it would be async
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