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SooHow Cheng
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Got NDR 550 5.1.0 550 #5.1.0 Address Reject, how to solve it?

This is using MS Office 365. My user try got this ndr while he tried to send an email to this abc@def.com (fake domain). Please see the messages,


Office 365 received the message that you specified, but couldn't deliver it to the recipient (abc@def.com) due to the following error:

Error: 550 5.1.0 550#5.1.0 Address Rejected

A non-delivery report (NDR) message was sent to <the sender>. The NDR might provide more details about why the email message wasn't delivered and how to fix the issue.

How to fix it

As the sender to follow the instructions in the NDR to fix this issue. The NDR might also include specific information for email admins. If the sender is unable to fix the issue, ask them to forward you the NDR and then follow the guidance for email admins.

Message event

Event    fail

detail    Reason 550 #5.1.0 Address rejected



              OutboundProxyTargetIP: 103.138.xx.xx

               OutboundPorxyTargetHostName = mx.def.com

What shall we do to be able to send mail to the above recipient?

Thanks in advance

Microsoft OfficeMicrosoft 365* Exchange Hybrid* mailserver

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Last Comment
SooHow Cheng

8/22/2022 - Mon

You have to contact the administrator of the server. He is in full control over the mail flow, and also hence, what email addresses are allowed or not.
Dr. Klahn

Hard to tell what happened here, as 550 is a generic code used for many different things.  The problem is not, however, that the delivery address does not exist.

Your domain, MTA or CIDR block may have been banned by the remote admin.

Your domain, MTA or CIDR block may be on spam source list such as Spamhaus.

And it may simply mean that the recipient doesn't want email from the sender and has put the sender on their private reject list.  This would be the one I'd suspect.  A phone call from the sender to the recipient is in order to find out what the situation is.

David Favor

Expanding Dr. Klahn's comment...

All SMTP codes have arbitrary meaning... or said differently, each MTA + MTA config can be modified in any way.

So non-250 (success) SMTP codes are always suspect... meaning you never really know what they mean.

Provided a real From: or To: address for debugging might be help + might not.

As @Kimputer mentioned, the only way to answer your question 100% correctly, is to contact the def.com MX owner asking for them to analyze their logs + provide you more detail... which might or might not produce good results, as usually requests for this detail are ignored.
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SooHow Cheng

Hi all,

Thanks for the advise. I will contact the owner.
Scott Fell

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SooHow Cheng

Thanks experts, it works