Error message - invalid domain name

Invalid domain name means one of two things, either your DNS server can't resolve the domain name to be able to send the mail via the MX record (Mail Exchange Record) or it is in fact an invalid domain name (it does not exists or no longer exists). The domain name would be the part of the mail address after the "@" symbol. One thing you may want to check is the DNS configuration in the mail server. Power outages can do some goofy things to files if they are open when the power goes out. If the file is corrupted, it may not be finding your DNS server and therefore would not be able to resolve any domains.

Hope this helps. If you need more assistance, just let me know.