connection to server pop.mail.yahoo.com timed out

First thing to do would be to verify your server settings. According to Yahoo's help page (https://help.yahoo.com/kb/SLN4724.html) they should be this:

Incoming Server Type: POP3 Incoming Server Name: pop.mail.yahoo.com Port: 995 User Name: <your email address> Connection Security: SSL/TLS Authentication method: OAuth2

SMTP Server Name: smtp.mail.yahoo.com SMTP Port: 465 Connection Security: SSL/TLS Authentication method: OAuth2 User Name: <your email address>

Even though Yahoo doesn't mention it, I believe they now require the use of OAuth2 for authentication, and I indicate it above.

To force an OAuth2 re-authorize you may need to take some or all these additional steps:

- Make sure cookies are allowed (<Settings | Privacy & Security | Web Content | Accept cookies from sites>);

- Delete all cookies for that account (<Settings | Privacy & Security | Web Content | Show Cookies>);

- Delete all passwords for that account from saved passwords (<Settings | Privacy & Security | Passwords | Saved Passwords>);

Now restart Thunderbird. You will be asked for your credentials, an OAuth2 token will be set, and hopefully they will be accepted and mail retrieval will resume.

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If you have multiple Yahoo accounts set up in Thunderbird, there may be more hoops to jump through. But let's put that off until you try the above.

Do you have multiple Yahoo accounts set up in Thunderbird? If so, this article may apply to you:

https://support.mozilla.org/en-US/kb/thunderbird-and-yahoo

Please pay special attention to the section titled, "Configure a Thunderbird preference to make multiple accounts work". The gist of it is that you have to take some special measures so that Yahoo's new security protocols are able to keep your various accounts separate during authentication.