I keep tripping over things... trying to transfer the previous two quarters to SM folders with the names 2q and 3q.

In Tbird, I drag the 2q stuff from its local folder to its inbox.

That stuff shows up in the SM inbox. I drag that to the SM 2q folder. I check that folder to confirm it really happened.

Back in Tbird, I drag the 3q stuff from a local folder to the inbox.

That also appears in the SM inbox, but when I drag it to the SM 3q folder, everything I just put in the 2q folder has disappeared!

First, I have to presume that somehow I caused this. But if it's not me, this would make SM truly unusable. Please set me straight and thanks again.

I've assigned my SM emails to a local folder, by the way.