What is the authentication method on the incoming and outgoing servers, for both accounts? What is the antivirus, and is there a VPN? What is the selection in Preferences/General/Network & Disk Space, Connection?

Hi sfhowes, thanks for getting back to me but I think the problem might not be with Thunderbird. I tried to configure 2 other 3rd party email apps and had the same issue with both..."can't connect to server". I'm not having a problem on my other MacBook, both gmail and yahoo work fine in Thunderbird so it's not a network connection. It's making me crazy!

If it's not working with other apps, check that the account settings are correct (especially authentication method, which should be OAuth2 for gmail and Yahoo), and that no external app such as antivirus is interfering. The Connection setting should be 'no proxy'.

Both accounts are set to OAuth2 and I've had Norton running for years. Where would I find the connection setting?

Preferences/General/Network & Disk Space, Connection. To test the impact of Norton, it's recommended to run OS X in safe mode. At the very least, the TB profile folder should be added as an exclusion.

I've also tried to set up my existing email account: File-New-Existing Email Account and I get this message: "Thunderbird failed to find the settings for your email account.

OMG! It was the freakin' proxy setting. I have no idea why that changed, everything was fine until about 4:30 yesterday. You have saved my sanity...thank you, thank you, thank you!