RETR command problems

Went back & triple-checked all basic settings, & noticed that the ATT email pop & smtp settings have been changed again, so reset as follows:

pop.att.yahoo.com -changed-to- inbound.att.net
smtp.att.yahoo.com -changed-to- outbound.att.net

However, after these changes & having again tested both sending & receiving of all email-addresses, the issue remains: all 4 email-addresses send fine; 3 email-addresses receive fine; 1 email-address still gets the error:

"The RETR command did not succeed. Error retrieving a message. Mail server inbound.att.net responded: problem retrieving message."

(which is identical except it's now "inbound.att.net" instead of "pop.att.yahoo.com")

Hello @w__ 

I'm sorry you're having some issues with one of your email accounts. Our team can help you with that. Please send us a private message by clicking here with your name, phone number and the best time you can be reached.

Thank you,

Dmitriy

Thanx for the responses from Dmitriy, Community Manager & Sue C, AT&T Social Media Manager.

Private message replies have been sent... but really don't understand why one email address won't send to my home email client, why I'm forced to use the--frankly, utter crap--Yahoo/neo/AT&T online email site.

These private message contacts are doing no good.  Francisco R, AT&T Social Media Manager, can do nothing but attempt to blame my email-client. 
Shall we consider the logic, or rather the lack thereof?... THINK!  
 
4 email addresses, all of the same main AT&T account.  All 4 are set up identically in the email-client.  All 4 work perfectly well for years, nearly a decade.  There are NO changes made to the settings. 
Suddenly one--ONE-&-ONLY-ONE--of the SAME-AT&T-ACCOUNT email-addresses is not receiving mail from the AT&T/Yahoo email-servers. 
The AT&T/Yahoo email-servers still allow the email-address to send, 'outbound.att.net', via the email-client. 
But the AT&T/Yahoo email-servers do NOT allow the email-client to receive, 'inbound.att.net'. 

If the email-client were malfunctioning, it would not be receiving, 'inbound.att.net', the other 3 identically set-up, same-AT&T-account email-addresses! 
STOP TRYING TO BLAME MY EMAIL-CLIENT! 

Someone PLEASE contact someone higher up who has the capability of finding out what is going on with the AT&T/Yahoo email-servers!... PLEASE! 
w
t***@sbcglobal.net (1-of-the-3 email-addresses still able to receive via my email-client) 
n***@sbcglobal.net (the email-address that won't receive via my email-client) 

[Edited to comply with Guidelines]

Thank you for your response, Brandon P, U-verse Social Media Manager: 

I do not want this to go off into yet another false direction.  The first false direction, disproved through obvious logic, was that there was some issue with my email-client. 
Let us, please, NOT, go off in a second false direction getting bogged down in separate problems that MULTITUDES of people are having with Yahoo/NEO email & Yahoo/NEO Group websites. 

There is some decision that has been made, way up the ladder, at some more fundamental/upper level, not to allow the n***@sbcglobal.net email to be delivered to my email-client... WHY??? 

PLEASE, keep going 'up the ladder'... I would think there is some mechanism to stop an email-address from being allowed to receive email on their home email-client... what triggers this?, why would this be done?, why has my n***@sbcglobal.net email been flagged as: "DO NOT SEND THIS EMAIL"? 

Remember the n***@sbcglobal.net email error message?: "The RETR command did not succeed. Error retrieving a message. Mail server inbound.att.net responded: problem retrieving message." 
I repeat: "[AT&T/Yahoo] MAIL SERVER inbound.att.net responded: problem retrieving message" 

Meanwhile, Brandon P, I am going to answer your most recent questions, however, again!, let us NOT go off in more false directions!, OK?, please!? 
Technically, yes, I am, "able to login at mail.yahoo.com with the n***@sbcglobal.net". 
HOWEVER: THIS IS NOT A VIABLE SOLUTION! 

As to: "Can you send and receive emails from the mail.yahoo.com website?":  From the mail.yahoo.com website, the AT&T/Yahoo/NEO mail-servers randomly will or will not allow certain n***@sbcglobal.net email to be forwarded--or may suddenly later allow an email to be forwarded that earlier would not send--for instance to t***@sbcglobal.net, one of the other 3 same-AT&T-account email-addresses still functioning fully w/ my email-client.  The problem is, the mail.yahoo.com website, which is run under Yahoo/NEO--the same as utter crap Yahoo Groups--is SO badly coded, is SO buggy, locks up SO often, has SO many problems, that it would seem impossible to determine which--probably-all--of these RANDOM, INTERMITTENT failed forwards are due to the crap website, or which--if-any--are due to this n***@sbcglobal.net email issue.  There's certainly no rhyme or reason to what email will forward, & what won't, or when the same email that wouldn't forward before, suddenly will be allowed to forward; virtually identical email from the same sender will or won't be forwarded, or won't be allowed to be forwarded at one moment, but may be allowed to be forwarded the next moment, or the next day.  Yes, 'Sends' which aren't 'Forwards' will generally actually be sent more often than the forwards, but are still at least occasionally problematic. 

Again, PLEASE, keep going 'up the ladder'... I would think there is some mechanism to stop an email address from being allowed to receive email on their home email-client... what triggers this?, why would this be done?, why has my n***@sbcglobal.net email been flagged as: "DO NOT SEND THIS EMAIL"? 

w

t***@sbcglobal.net (1-of-the-3 email-addresses still able to receive via my email-client) 
n***@sbcglobal.net (the email-address that won't receive via my email-client) 

I have begun having the same problem, when I don't get "Failed to connect to server inbound.att.net."  I have been using, successfully, the same email program, "PostBox", for several years on my desktop and have made no changes.  I installed "PostBox" on my laptop and tried downloading my mail there.  It started downloading mail from 2013!  I can still download and send mail on the Yahoo! website.

I have been having the exact same problem for over a week. I have 6 email addresses and all but one receives emails in Thunderbird. No matter what I do-I have even deleted the account and re-added it-I get the same message. "The RETR command did not succeed. Error retrieving a message. Mail server inbound.att.net responded: Server error - Please try again later.". The Thunderbird help forum is of no use. I would appreciate any solutions that may work. You may send an email to wzcohenmd@att.net.