I tried this (and found that all but one of the accounts managed by Thunderbird were set up exactly as you suggest) but, just to be certain, I re-did it.
After a day of use I am still seeing this, but it's not consistent enough (naturally) to lend itself to debugging. IOW, I don't know if it's a Thunderbird or Dovecot or other issue.
Looking at the mail logs I don't see any hint of the problem. However perhaps there's other debug capabilities I'm missing, so if anyone has a recommendation, I'm among the willing.
Note that there has been an ongoing certificate mismatch issue which in Thunderbird is resolved by the "Remember mismatched certificate" extension. Thunderbird only presents the mismatched certificate dialog when first logging in, anyway, so this is probably not related. The reason for the mismatch is that I was lazy and haven't established a cert for each mailserver uri.
/m
M.-A. Lemburg wrote:
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.