Does the password work if you login at the Eastlink webmail page? Are you trying to send through mail.eastlink.ca on port 465, SSL/TLS, normal password, and full email address for the User name?

Thanks for your response. Away from home for several days, but can clarify as follows, and hoping for more possibilities:

Does the password work if you login at the Eastlink webmail page?Yes

Are you trying to send through mail.eastlink.ca on port 465, SSL/TLS, normal password, and full email address for the User name? Yes - all

When you say 'not blocked by antivirus', keep in mind the only way to rule it out is to run in Windows safe mode. For W10, Shift+Restart, Troubleshooting, Advanced Options, Startup Settings, Restart, 5. Also, check your firewall, especially if it's not the built-in Windows one.

Thanks for your help! "Full email address for username" seems to have done it :)

After changing my gmail password, T'bird couldn't receive or send emails even though I had updated the password (Options..Options..Security..Saved passwords). Mail worked fine via browser (gmail.com) with new password.

In desperation, deleted the account in T'bird and went thru setup again (Options..Account settings..Account actions..New account). Still failed to receive/send at first, but suddenly could receive emails again. Sends via smtp.gmail.com failed every time, even when correct new password entered at prompt.

Finally discovered that my imap authentication method had been changed (by gremlins?) from "Normal password" to "OAuth2". Since receive mail now worked, I changed smtp authentication to "OAuth2" and send worked fine too.