An 'unknown reason' is hard to fix. Did you add something like a VPN or new security/antivirus app? A restart of the modem/router might help.

Nothing has changed to my knowledge.

I did a reset of the modem when the problem was seen but it made no change. The system is a laptop and I connected it to another network but the problem was still seen. I also installed Thunderbird on a desktop system that had a clean install of Windows 10 and configured the account and the issue was there as well.

This is an intermittent problem that's mostly seen when replying to or forwarding a message that's been received.

Is there any way to log the communication between Thunderbird and the smtp server which might give insight into the issue?

Logging is explained here: http://kb.mozillazine.org/Session_logging_for_mail/news

To test if this is a TB or Cogeco problem, add your account to the W10 Mail app, or similar, and see if the error persists.

It's also possible to send through a different provider's smtp, e.g. the gmail smtp, if you have an account.

I've sent messages with two other smtp servers... Gmail and another ISP I have access to. I had no problem with either of these.

I configured Pegasus Mail with the account and have had no problem with the Cogeco SMTP server.

I will continue troubleshooting using logging over the next few days.