Unable to send or receive email from server in TB 78
Suddenly, I am unable to receive or send emails using Thunderbird with Yahoo small business. I have been using 78.4.2 with Catalina 10.15.7 on my MacBook Pro since last November without problems. Then, two days ago after functioning normally for hours, my outgoing attempts received a dropdown error message “unable to connect with SMTP server”. Same type of error message when I tried to get messages “unable to connect with POP server”. I am still able to receive and send email on my iPhone, but not on the laptop which makes me suspect that it is something on the laptop rather than on the Yahoo small business side. I was also able to send successfully from the same account on a second machine. There is no firewall or VPN involved. I double-checked the pop and smtp configuration that Yahoo requires and have continued with the same result. I then uninstalled Thunderbird and did a fresh download and install of TB 78.7.1, and created a new profile. Same result. I am still able to use webmail, so all is not lost, but feel like I’ve exhausted the possible fixes I can think of, and am no closer to a solution.
Chosen solution
I switched authentication back to Normal Password and restored the app passwords, but left Proxy setting on No Proxy; also used detect Proxy setting. All works properly now. Appears that hitch was in the Proxy setting. Many thanks for stepping me through the troubleshooting process. Still a mystery as to why the handshake between TB and bizmail.yahoo suddenly changed.
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What's the authentication method for the incoming and outgoing servers, 'normal password' or OAuth2 (recommended)?
Authentication is Normal password; that's what Yahoo asks for. I tried OAuth2 and Yahoo kicked it back.
Change it to OAuth2 for incoming and outgoing, remove the passwords from Saved Passwords in Preferences/Privacy & Security, allow cookies in TB Privacy & Security, restart TB, enter the account password (not app password) in the OAuth browser window when prompted.
I enabled cookies, erased the app passwords from saved passwords, and then reconfigured incoming and outgoing servers to OAuth2 (although the only way that I was able to do this was by acting as though I was setting up a new email account -- OAuth2 was not in the drop downs for editing in Account Settings) -- this took in the incoming settings, but Normal Password stayed in the outgoing. I also used the bizmail.yahoo.com account password in configuring.
When I reopened TB there was no OAuth window, however the same error banner came from the upper right part of screen, saying that could not connect to server. This happens immediately when TB opens, and then each time I try to get messages.
What is the proxy setting?
https://support.mozilla.org/en-US/questions/1306336
If you can't change the outgoing authentication to OAuth2 in Account Settings, it can be changed in Preferences/General/Indexing/Config. editor by changing mail.smtpserver.smtpN.authMethod to 10 (right-click, Modify), where N = 1,2, 3 or... depending on the account.
Progress of sorts -- I made the edit so that OAuth2 is the authentication for both incoming and outgoing. Then I checked Proxy -- it was set for system Proxy, and I changed it to No Proxy. Then I tested receive and send. It was successful reaching the server, but I got the error message "The server does not support selected authentication method."
I think I misread the Yahoo Business article. While the incoming IMAP server, imap.mail.yahoo.com, does work with OAuth2 in TB, the POP and SMTP servers are slightly different, e.g. smtp.bizmail.yahoo.com. The bizmail servers may support OAuth2, but I don't think those servers and OAuth2 are supported in TB (gmail, free yahoo, aol, yandex and a few others are supported). So, I guess you can use OAuth2 for IMAP, but must use 'normal password' for POP and SMTP, and an app password instead of the regular account password for POP and SMTP.
Chosen Solution
I switched authentication back to Normal Password and restored the app passwords, but left Proxy setting on No Proxy; also used detect Proxy setting. All works properly now. Appears that hitch was in the Proxy setting. Many thanks for stepping me through the troubleshooting process. Still a mystery as to why the handshake between TB and bizmail.yahoo suddenly changed.