Emails sent to us from specific sender are returned to them

If one company is reporting that they are receiving the following message, is the problem on their end?  (They are asking us to fix the situation)

"This is the Postfix program at host xx.xxx.xx.xxxxx.com.
I'm sorry to have to inform you that your message could nto be delivered to one or more recipients.  It's attached below.
xxxxx@xxxxx.com: [xxxxxx.com]: Name or service not known"

Thanks!
dhullettAsked:
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Marc ZCommented:
If you are receiving mail from others, it is their issue.

Perhaps they simply have a comma where the period is supposed to be. Perhaps they have the entire email address incorrect.
Are you able to Send them a message?  What happens when they hit a Reply to your message?

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dhullettAuthor Commented:
we are receiving mail from everybody else....and yes they receive our messages.  Replies to our messages also get bounced back to them.

we have worked with them for a long time, and this has just started (my reasoning that they do not have our email address incorrect)

thanks!
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Marc ZCommented:
hmm, then it sounds like their mail server is mucked.  Again, not your end, theirs.
It looks like their Postmaster somehow disabled your address.  Do you have an alternate address they can try?  Do they have a whitelisting program?

Is the Host name in the error report their host?
"Postfix program at host xx.xxx.xx.xxxxx.com." this is the host that needs fixing.
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dhullettAuthor Commented:
Yes, the error message contains "this is the Postfix program at host xxx.xx.xx.xx.com" (their site), I will contact their admin re whitelist, etc.
Thanks for the help!
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