Error - Thunderbird - Your message was sent but a copy was not placed in your sent folder

 Error - Thunderbird - Your message was sent but a copy was not placed in your sent folder

"Your message was sent but a copy was not placed in your sent folder (Sent) due to network or file access errors.  You can retry or save the message locally to a local folders/Sent."

Plus: "Status: Copy failed"

Problem:
When sending an outbound email
Likely, you had just recently completed other Outbound SMTP email changes or the message you are trying to send was previously written before SMTP changes were completed.

Solution:

At least for me, the solution was to close and re-open the Thunderbird client.
Then re-compose a new test message.

(and, I am assuming you are using IMAP, where message are synced to the client and to the server - recommended.)

This fixed my problem.
Other articles recommended rebuilding folders, going into the Thunderbird's settings and changing how Outbound emails are stored (storing them locally), turning off Oauth2, turning off TLS, and other nonsense.