For about a week I have been unable to retrieve my email. I use Mail and (try to) connect to ATT/Yahoo. I get the message "The POP server “pop.att.yahoo.com” rejected the password for user..." Which gives me a blank box in which I re-enter my password. That fails and the message appears again, with the box blank again. Ad infinitum. I googled the problem and others have had it.

Try logging into your ATT mail account directly and see if your password works or has mysteriously changed.

To do this, in your favorite browser, go to www.att.net (not .com), click on the "Mail" button and enter your user name and password. If this works, then the problem resides in the Mail application and/or your setup. If this does not, contact ATT to reset your password.

I am also a little confused when you say you try to "connect to ATT/Comcast DSL." These are two different providers. Did you switch and keep pieces of both?

I have had a similar problem with an ATT/Yahoo account. Apparently their server gets messed up and does not recognize the passwords. Generally waiting a few hours and trying again solves the problem. There is absolutely nothing you can do on your end in this case.

Try visiting https://mail.yahoo.com/ instead.

AT&T's move to yahoo for e-mail really irks me. In their effort to compete with google, yahoo has become a haven for spammers. And their web-mail page is also loaded with tacky advertisements. So here we are —loyal paying AT&T customers —being bombarded both ways (with spam and ads).

[I only visit that site to add more domains to my spam filter.]

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edit/ oh wait, now i got you... so you're using port 995 with SSL enabled i gather?

I remedied a similar problem with Verizon by reinstalling the applicable OS X Combo updater. (I note that the problem recurred and that I had to reapply the Combo several times over a period of time before it was remedied for good.)

Sorry, I meant ATT/Yahoo.

I tried accessing from att.net but the login didn't work. "The information you entered does not match our records."

I tried accessing from https://mail.yahoo.com/ but there is no response, just sat there like a freeze or stall, no activity.

Had no luck resetting my password via att.com. "Check to be sure the information entered is correct, and try again. "

Please don't tell me I have to call ATT.

One thing…I can still send.

That's odd too then. Might be a DNS issue.
It should jump to a login session address.

[try deleting all att.* and yahoo.* cookies maybe]