Delivery has failed to these recipients or distribution lists

I get the following error only when replying to received emails and only from one domain,  If I create an email and send directly to the mail recipient every thing works correctly.

Delivery has failed to these recipients or distribution lists:
Microsoft Exchange has been trying to deliver this message without success and has stopped trying. Please try sending this message again, or provide the following diagnostic text to your system administrator.

The following organization rejected your message:

Diagnostic information for administrators:

Generating server: #< #4.4.7 smtp; 250 2.1.5 <>... Recipient ok> #SMTP#

The generating server is a smart host that we forward all mail through because our ISP can't seem to properly create a PTR record for us.

Sounds like it is on their or the ISP side and not mine.

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Mahmoud SabrySenior IT Systems EngineerCommented:
are you sure that reply(when using reply button) to address is exactly like the address when you write it by hand ?
lrmcenterAuthor Commented:
Absolutely the same
To be 100% certain it's the ISP you're relaying through and not your configuration, do you have another external mail account you can use to relay through?

It is odd that you can compose a new e-mail to but you cannot reply to

I would also run MX Toolbox ( to check your mail server as well.

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lrmcenterAuthor Commented:
This is weird ... My MX record has disappeared but my PTR is correct.
lrmcenterAuthor Commented:
Re-created my MX record and things are working now.
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