My POP3 email account in the Mail App no longer working

Per chance did you enable 2-Factor Authentication (2FA) with your Yahoo email account? If so, you will need to enter an application-specific password, in place of your "normal" password to access this email account via the Apple Mail app.

Ref: Using app-specific passwords - Apple Support

Note that this article covers how to use your Apple iCloud email on a non-Apple Mail app. It is not exactly what you want to do here. Yahoo email, may or may not, offer 2FA security. I'm only guessing that it might, and if that is the case, they should offer an option to you to create this app-specific password.

I found this link for Yahoo that may prove helpful:

• https://my.help.yahoo.com/kb/account/generate-third-party-passwords-sln15241.html

POP3 is an older mail retrieval protocol, and a number of email providers don't even support it, in favor of using IMAP instead. I'm not 100% familiar with Yahoo mail, but I'm assuming, by what you provided so far, that it still supports POP3 ... correct? That is, you can still access it as a POP3 account from other devices, just not the Mail app on this particular Mac.

The reason I had you look into 2FA is the typical issue we see here is folks not able to log into their email accounts, and that was the prime suspect.

Let's give this another look at. I suggest that you enter your Yahoo email address into the following Apple Support article. It should result in the correct email (incoming & outgoing) mail servers required for their services.

• Ref: Mail Settings Lookup - Apple Support

The results I got were only for IMAP, and they were: imap.mail.yahoo.com (incoming) & smtp.mail.yahoo.com (outgoing). Not finding similar results for POP3, either means they no longer support POP3 or the Apple link doesn't look for POP3 accounts at all.

Whatever the results you get, these should be the proper email server addresses to confirm in your AT&T/Yahoo mail settings. It should also include the proper ports to use a secured connection to them. I suggest double-checking them with your settings. It may just be possible that everything is correct, but Yahoo was experiencing an issue at their end.

If they do check out, and you are still having this issue, the next step is to use Apple's Connection Doctor to see where the communication to the Yahoo servers is failing. Please post your results. Ref: Use Connection Doctor in Mail on Mac - Apple Support

Restoring from a Time Machine backup solved the problem. The offending account was back immediately, though Mail was very flaky for a few hours, refusing to put items into the trash, or connect to other accounts. All seems fine as of now.

Yahoo has gone to this too...

https://help.yahoo.com/kb/access-yahoo-mail-third-party-apps-sln15241.html

Just an FYI. As I mentioned to the OP, POP3 & IMAP are two different email protocols for receiving messages. The real difference between them is how they fundamentally work.

To use an example:

• For POP3, when an email client accesses the mail provider's server all the messages located there are downloaded to that client. If the user has more than one client (another computer, smart phone, etc.) to access their messages from, only the first client would receive all of the messages. The next client that they use would find their mailboxes "empty." This has been the major complaint from POP3 users and why IMAP was introduced.
• For IMAP, all messages are retained on the mail provider's server. A user with multiple clients can access all of the messages there. However, if one client deletes a message, it will not be available to the other clients.

For SPAM, there really shouldn't be any difference whether you use a POP3 or IMAP account, unless the email provider added additional anti-spam features to one of those types of accounts. It is really up to the email provider to offer these types of services since they administer these servers ... and this would be known as "server-based" filtering. On the other hand, you will find a number of third-party apps available for your Mac to control SPAM that "got through" the server filters. One I would suggest you consider is SpamSieve. (Disclaimer: I do not work for C-Command Software, but have used SpamSieve for a number of years and it has proven very useful in reducing the number of SPAM messages in my mailbox.)

For security, regardless of whether you use a POP3 or IMAP account, the email provider may offer 2-Factor Authentication. This is additional security for your email account access credentials (username & password), not the email servers themselves. However, as you know, you need to use these credentials to access those servers to send or retrieve messages.

Have you tried setting up the account afresh from the System Preferences Internet Accounts pane?

I had the exact same issue. After trying for 2 days to get the POP Yahoo working I gave up and then switched to IMAP setup. Not sure why it stopped working. About two months ago had to setup the new Yahoo security password system and it worked fine but then just stopped. I have now got used to the IMAP format although I preferred POP as it stopped more of the spam getting through.

Yahoo mail server offers better spam filtering when using POP vs IMAP. It has always been that way for some reason. With IMAP all the mail labeled spam is sent through to the Apple Junk Folder but with POP it is caught and kept in the spam folder on the server. Also when I try to block email sender in Apple mail and want the mail to go directly to trash it just stays in the junk folder no matter what setting I try for some reason running Big Sur.

I had (still have) a variant on this, same system. Mail sporadically stopped connecting to the server last week, then permanently failed. ISP suggested backing up mail, deleting the offending account, then recreating it. Straightforward until when attempting to set the new account got the message "Unable to verify account name or password". Trying to set up the account from the Internet Accounts pane then resulted in "This account already exists." at which point I remain stuck, rebooting has no effect. I have no option available of account type. There is no problem a the server end, my ipad picks up the mail fine, as does webmail, but I find neither very convenient

My ISP is internode (in Australia). 5 accounts in the left column, iCloud (unused),my professional (Exchange) account, and 3 internode accounts. Of these the only active one is the billing account from internode, which is working fine. The deleted account (which somehow still exists) is not there. I am considering using Time Machine to reinstall the system from before the difficulties.