I'm running into a little problem with my mail server. A client of mine can't send e-mail to a certain domain. It is returning the e-mail with the following error:

> > Delivery failed
> > 550-Verification failed for <e-mail address omitted>
> > 550-It appears that the DNS operator for domain omitted
> > 550-has installed an invalid MX record with an IP address
> > 550-instead of a domain name on the right hand side.
> > 550 Sender verify failed

I checked the MX record, and it was pointing to the mail server's IP Address. I changed it to point to the mail server's domain name instead. But when the client trys to send e-mail it still returns this error.

Is it possible that the receiving mail server has the old MX record's information cached? Is that usual? Any ideas how I can look into this error more, and see if I am missing something? Thanks in advance for any help.