I do not know how related this is,
or how relevant this is....
After last month upgrading to Fedora-30, and as a part of that, upgrading to Thunderbird
68, I sometimes experience the following:
* after sending a message, the copy that is put in the Sent folder is bolded/highlighted
like an unread message in the Inbox;
* after sending the message, two copies are put in the Sent folder, and one is
bolded/highlighted like an unread message in the Inbox.
These happen both in yahoo accounts and in the comcast account.
These each occur only sometimes.
I have not noticed any pattern to when these occur.
Bill.