As far as I can tell, this question doesn't already exist here but if it does then I apologize.
As of January 1st our company name has changed and, as such, our e-mail addresses have changed. The switch has gone off well with the exception of a few weird "5.1.1 User Unknown" errors that some external senders are getting when they attempt to send e-mails to us.
I've spoken to an external consultant to get their take on it and they seem to think the recipients have stale DNS records. The MX records to our server has existed on our external DNS for over a month - the new domain was set as our "Primary" domain on January 1st.
I've managed to get a couple of the error reports from the recipients and they look like this. Any help would be appreciated with this matter
The following recipient(s) cannot be reached: 'User Name' on 13/01/2012 11:07 AM 550 5.1.1 <email@example.com>... User unknown
EDIT : The domain listed above is the "new domain". E-mails sent to the old domain still send without issue. I can also verify that the policy has been set to that user and that the new e-mail address exists within their "E-Mail Addresses" tab.
EDIT 2 : The receiving mail server is running VamSoft ORF for anti-spam. Neither Vamsoft ORF logs or exchange logs show any sign of the e-mails getting to our domain, however when they use the old domain they go through without issue.