You need to alter server setting to force the email address/mail account to point o the correct servers. Yahoo servers have different requirements and use Authentication Method: OAuth. If by any chance you have accessed yahoo cox webmail account and set up 2 step verification and created an app specific password then switch it off because OAuth does not use it and yahoo will get confused over what password it expects.

In Thunderbird First in Settings > Privacy & Security - enable cookies. select 'Accept cookies from sites' Then scroll to Passwords section - click on Saved Passwords and Remove the stored password for both pop or imap and smtp. Now access Account Settings and alter both the incoming and outgoing server settings: You do not mention whether your current cox account is set up as Pop or Imap, so you need to adjust as appropriate. eg: If currently set up as Pop then use the Pop server settings.

IF POP incoming server settings:

• server name: pop.mail.yahoo.com
• port 995
• Connection SEcurity: SSL/TLS
• Authentication Method: OAuth2
• user name is full email address

IF IMAP incoming server settings:

• server name: imap.mail.yahoo.com
• port 993
• Connection SEcurity: SSL/TLS
• Authentication Method: OAuth2
• user name is full email address

SMTP Outgoing

• server name: smtp.mail.yahoo.com
• port 465 -
• Connection Security: SSL/TLS -
• Authentication Method: OAuth2 -
• user name is full email address.

REstart Thunderbird Then you will get a prompt by yahoo for email address to enter normal password and click on Next - follow instructions to allow Thunderbird access to server.

I went through all the steps but still can't get my emails to transfer to Thunderbird. Also, on the SMTP Outgoing steps Thunderbird does not have the "0Auth2" listed in "Authentication Method". Thanks Stan

Went thru all the steps above and still can't send emails and I get this message:

Sending of the message failed. An error occurred while sending mail. The mail server responded: Request failed; Mailbox unavailable. Please verify that your email address is correct in your account settings and try again.

What a terrible transition.