I've just aquired a new client and I'm hosting email services with them. during the changeover and data migration the old vendor exported all the mailboxes to pst files and gave them to me. I've imported all their old emails successfully into their new email boxes. The problem is whenever they reply to one of the old emails the email immediately bounces back and says:
The message could not be delivered because the recipents destination email system is unknown or invalid. Please check the address and try again, or contact your system administrator to verify connectivity to the email system of the recipient.
All new emails are coming in and all new emails are working with replies and new. The old email domain is being forwarded to my new exchange server. What can I do to make the replies work on the new emails system?