Here's a work-around for the problem that appears to work and maybe also hints at the root of the problem:
If you setup the Thunderbird account to "Place a copy in:..." "Other" and then select the Sent folder of the account (rather than just selecting "Sent Folder on: ..."), the problem appears to go away.
I can't tell for sure, but Thunderbird seems to send the copy using a different connection than the one used for sending the message itself if you set things up in this way.
The email still gets copied to the same Sent folder, so there's no change there.
-- Marc-Andre Lemburg eGenix.com
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