Account inactive notice

I've received complaints from more than a handful of my site users that say they cannot send me email. I seem to have no problem receiving regular emails to my mailbox (ie. [email protected]) but certain users get the following message when they try to send me an email:

This is the email server for the foocompany.com domain.
...
<[email protected]>: Permanent Failure: Account Inactive.

I've tripled-checked through SmarterMail that my account is valid, and have others send me test mails successfully. This is not relegated to just one ISP either...the users that have notified me of this use (for example) airmail.net, skybeam, etc. and the error is not intermittent either; they consistently get this error when attempting to send to me. What could possibly be causing this? Is it their ISP caching bad info somewhere or something else in the chain? Any help is GREATLY appreciated as some of these users are critical customers!

 

The domain is tanktransport.com

I greatly appreciate the help!

 

There is only one MX record on the domain and it points to our mail server, so that's fine. No extra name server records...

Do the users who get the error message have anything else in common (that you know of)? Same ISP, email service, etc.?

 

I'm going to see if I can gather any more information from the users who are seeing the error. I know that they are not using the same ISP, but not sure yet if there are any other commonalties. I'm glad to know that it's not something on this end, and hopefully this will be something they can resolve with their own providers. Thanks much for the quick replies and assistance!