Starting yesterday my users have begun receiving the following when sending to certian domains.
The following recipient(s) cannot be reached:
firstname.lastname@example.org on 7/24/2008 10:02 AM
There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
<mail.educationalproducts.com #5.5.0 smtp;550 use the proper MX please>
I've checked our DNS and MX records at mxtools and dnsstuff and they all seem to be in order. Any suggestions would be appreciated.