What's the antivirus? Several of them, such as Avast/AVG, Bitdefender, ESET, TrendMicro etc. are known to block connections without careful attention to their email settings. Do you connect through a VPN?

Thanks for your reply. I'm not using any anti-virus application and I don't connect through a VPN. Nothing has changed since it all was working fine a few days ago.

What is the authentication method on the gmail account? If it's not OAuth2, Google may have turned off access by 'less-secure apps' and blocked the connection.

It never hurts, and often helps, to restart the router/modem.

Thanks for reminding me to reboot the modem and router. I just tried that but no change in the Thunderbird "Connection refused" errors. The authentication method is OAuth2 for Gmail. Just a regular insecure password for the rcn.com mail. Both accounts worked before and neither are working today.

Thanks for the reminder to reboot the modem and router. I just rebooted both and the error message is the same.

The authentication mode for the gmail account is OAuth2. It worked fine until today. The other account is an rcn.com webmail. Both accounts are getting the same "connection refused" error message.