Blacklisted by an IP address that does not belong to me

I got an email from one of my clients stating they cannot receive an email to a certain person.  Here is the message the recipient receives.

This email is being returned to you because the remote server would not or could not accept the message. The registeredsite servers are just reporting to you what happened and are not the source of the problem.

The address which was undeliverable is in the section labeled:
  "----- The following addresses had permanent fatal errors -----".

The reason your mail is being returned to you is in the section labeled:
  "----- Transcript of Session Follows -----".

This section describes the specific reason your e-mail could not be delivered.

Please direct further questions regarding this message to your e-mail administrator.

--Registeredsite Postmaster

   ----- Transcript of session follows ----- ... while talking to
>>> RCPT To:<>
<<< 550 Unable to add because host is listed on
550 5.1.1 <>... User unknown
>>> DATA
<<< 554 no valid recipients, bye
451 4.4.1 reply: read error from

My Spam filter uses the IP's and
The Exchange server is using 50.251.186.***

I cannot figure out how they claim that his email address is going to

Is this something on the senders end or my end?
Computer HelperWindows Server AdminAsked:
Who is Participating?

[Product update] Infrastructure Analysis Tool is now available with Business Accounts.Learn More

I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

Patrick BogersDatacenter platform engineer LindowsCommented:

If the sender is unable to send mail to one of your clients it means the sending mailserver has an issue with the blacklist.

In other words, if your servers werent configured correctly and ended on a blacklist you would not be able to send mail to the entire world.

Does this infosniper link rings any bells for you ?
Computer HelperWindows Server AdminAuthor Commented:
Turns out that I was misinformed. It was the other company that is trying to email my client. Therefore my client cannot receive their email because they have been blacklisted.
The message is that they can not add the recipient because the host presumably the source of the mailing is blacklisted.

The problem/difficulty is that you are presumably the admin of the "" domain is asked to address the cause of mail server rejected a sender from from relying through their system.

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
10 Tips to Protect Your Business from Ransomware

Did you know that ransomware is the most widespread, destructive malware in the world today? It accounts for 39% of all security breaches, with ransomware gangsters projected to make $11.5B in profits from online extortion by 2019.

Patrick BogersDatacenter platform engineer LindowsCommented:
Yes that is a correct assumption.
Computer HelperWindows Server AdminAuthor Commented:
Since that IP is blacklisted shouldn't I tell them to fix that first?
Presumably those who tried to send the message, received the NDR with the error you were provided.

This blacklist is auto-clearing after enough time has passed.
The IP is no longer listed.
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today

From novice to tech pro — start learning today.