As a Windows computer tech, what I would do in this case would be: '''1.''' [https://mzl.la/2lBfidM#w_backing-up-a-profile Backup the Profile folder]. In fact, you should do this regularly in any case. Backup the entire computer regularly. '''2.''' Remove that login information again from Thunderbird (Menu bar: Tools: Options: Security section: Passwords tab. Click "Saved Password" button, find and remove all entries associated with your e-mail address). '''3.''' Quit Thunderbird. '''4.''' Run a complete Check Disk, in which not only are the files checked, but also disk clusters are checked. [http://brucejohnson.ca/CheckDisk.html How to run a "Check Disk" in Windows] '''5.''' Run a Disk Cleanup. [http://brucejohnson.ca/DiskCleanup.html How to run a "Disk Cleanup" in Windows] '''6.''' Start Thunderbird, enter the passwords when prompted. Quit Thunderbird and start it again to see if things are working better. If the issue persists after this, then I recommend [http://brucejohnson.ca/ThunderbirdProfileNew.html creating a new Thunderbird Profile] to see if it works better. If the new profile works better, then http://brucejohnson.ca/ThunderbirdProfile.html copy your stuff from the old profile to the new one].

Thanks Bruce! Ended up creating a new profile to resolve the problem. Password manager was not creating credentials for the sending server smtp://doman-name but did for the imap://domain-name which is used for receiving. Now wonder if I could have manually created it to fix this. Richard