Email from one sender bounces back to her.

My email address is ****@sbcglobal.net.  Since mid March 2024 one friend who has lived in Germany for 30+ years and has a t.online.de email domain will send me an email that has bounced back to her ever since.  We regularly emailed back and forth all these decades until mid March.  She receives my emails just fine and can send and receive emails from every one else (including all her family in the USA.  The AT&T postmaster has been no help at all.  They had an issue at the end of March when I first requested assistance, I was told, but they claim that the issue is now fixed - even though it is clearly not.  She can use a gmail address and her email arrives just fine.  (But neither one of us uses gmail, so that is not a solution).  I have sent and resent the bounce back information to the AT&T postmaster as they initially requested, but they niether help nor even respond to my requests to resolve this problem.  Someone told me that the problem "looks like a block list on the Yahoo servers are stopping the email."  I have no idea how to fix this or how to get Yahoo or AT&T to solve this problem.  I have not been able to find a way to contact a Yahoo customer service person (without signing up for their premium service and this is something I should not have to do since yahoo/AT&T is causing this issue.  I get emails from everyone else that I email back and forth with.  The t-online.de postmaster emailed me back in March and noted two unlikely issues that might cause the problem on their end, but neither of those apply to my friend.  I would appreciate some assistance from anyone who can/will help. 

When your friend gets a NDR (non delivery report) or bounce-back message, what does it say about why the e-mail cannot be delivered to you?

I am not sure what the bounce back message says, but eventually every time she sends one, it times out.  I think t.online.de tried to deliver the email a certain number of times, then sends the bounce back message .  Someone mentioned to me that it looks to him like 

a block list on the Yahoo servers are stopping the email.   I do not have any filters set on my email settings and her address was not mistakenly put on the blocked email list (where suspected spam emails are placed).  I also do not have any extensions installed on my computer.  I had read that might cause an issue when trying to figure out the bounce back cause when it first occurred.  Thanks for any help you can give me.

Reporting-MTA: dns; mailout10.t-online.de
X-Postfix-Queue-ID: 71B9A2D995
X-Postfix-Sender: rfc822; *****@t-online.de
Arrival-Date: Tue, 14 May 2024 17:32:45 +0200 (CEST)

Final-Recipient: rfc822; ****@sbcglobal.net
Original-Recipient: rfc822; ****@sbcglobal.net
Action: failed
Status: 4.4.1
Diagnostic-Code: X-Postfix; delivery temporarily suspended: connect to
    al-ip4-mx-vip1.prodigy.net[144.160.235.143]:25: Connection timed out

That particular error looks like a communication error between t-online.de and AT&T's mail servers.  Normally, AT&T still accepts the SMTP connection from mails it has on its RBL, so this is different.

This is something that t-online.de and AT&T have to work out between them, and I don't know of any good way of making AT&T work this sort of issue.  Perhaps the sender would have more luck with t-online.de, but it sounds like they've reached the end of their rope too.

Thanks for your reply.  I reached out to the AT&T postmaster again, requesting that they contact the t-online.de postmaster to find a solution (since this bounce back issue has gone on since mid March, but once again they have failed to respond to me at all.  I understand I can contact AT&T headquarters to escalate this issue and file a "notice of dispute", so unfortunately, they will have to be my next steps.