[cobalt-users] weird email problem

["wcstaff" <wcstaff@xxxxxxxxxxxx>]

I'm having this weird email problem with one of our customers.
I can send them mail and they can send mail. But when someone outside tries
to send them an email, they get the following message:
The mail server at smtp1.ij.net could not deliver your message:

    denise@xxxxxxxxxxx:
    24.129.183.219 does not like recipient.

Remote host said: 550 5.7.1 denise@xxxxxxxxxxx  relaying denied
check your mail first or restart your mail session giving up on
24.129.183.219.

I cannot understand what may be causing a problem for someone to send email
to my customer. Like I said, my customer has no problem sending emails.
Any ideas?
Tom