DNSBL:RBL 521 - Emails blocked by ATT

That IP is a Microsoft address used for Outlook.com (Office 365).  You're probably the 8th or 9th person to report this in the last 10 days or so.  I'm incredulous that AT&T (or whatever blocklist they might subscribe to) would be blocking Microsoft, but there it is.

Not sure why your email bounced (did you get a bounce notice?) but it probably doesn't matter much as that dept seems to be a black hole most of the time. 

Hey @tonydi thanks for response , not sure but this is regular mailbox and used to send normal notifications . Emails are getting delivered to other external recipients.

However while while sending email to abuse_rbl@ i got below error, i tried sending from 3 different emails even from my personal :

Delivery has failed to these recipients or groups:

abuse_rbl@abuse-att.net‎ (abuse_rbl@abuse-att.net‎)
Your message couldn't be delivered because you weren't recognized as a valid sender. The most common reason for this is that your email address is suspected of sending spam and it's no longer allowed to send email. Contact your email admin for assistance.

🤣🤣🤣🤣   Sorry, but that's priceless. 

Try sending from some other domain/service.  Also, make sure it looks like a person created the email and not some bot.  Sigh.....

We're here to assist with your emails being blocked, @ksingh5268!

Thank you for the help so far, @tonydi!

You've already taken the correct course of action to resolve this by emailing using the email address you provided. As our ACE mentioned, try using a different email to send the email since you are getting failed delivery messages.

Don't hesitate to reach back out if you ever need our assistance again. Thank you for choosing the AT&T Community Forums!

CalebP, AT&T Community Specialist 

I tried sending using multiple email address , it is still getting bounce back

Weird, I just sent a test to the address and got this back a few mins later.  No idea why mine went through except maybe because it was just a plain email with no attachment.  Which is basically useless to those people.

====================================

Thank you for contacting the AT&T Postmaster Staff.

We investigate every block report and removal request we 
receive and take the appropriate action, of which you will
be notified. It is never the intention of the AT&T 
Postmaster staff to block legitimate mail. It is a rare 
but unfortunate occurrence that legitimate correspondence 
is blocked as a result of careful yet vigorous anti-spam 
and abuse countermeasures. We would like to rectify your 
mail delivery issue as soon as possible with your help. By
emailing this address you have already taken the first 
step in resolving this issue.

When the AT&T Postmaster staff receives a request with the
needed information requests are evaluated. If granted, the
block will be removed within 24 to 48 hours. There are 
situations when mail interruptions or blocks are in place 
outside the AT&T domain. In these cases, we will attempt 
to make contact with the party initiating the block and 
seek a resolution. If there is no change after 48 hours 
please include the current and complete error message that
you are receiving in your follow up email to 
<abuse_rbl@abuse-att.net>.

For more information about email best practices and mail 
delivery issues visit:

https://www.att.com/esupport/postmaster/

We reserve the right to deny or turn down a request if our
research shows the requested IP(s) being a violator of 
SPAM or abuse policies currently, or in the past.

We at AT&T thank you for your assistance in reporting this 
matter and appreciate your patience while we investigate 
this issue.   

AT&T Postmaster Staff
===============================

@ksingh5268 Are you still seeing this problem?  If so, I may have a way to approach this from the Microsoft side rather than wait for AT&T to get their act together.

What do you suggest, it is still blocked