I have a client running SBS 2003. They are hosting their own mail internallly on Exhange and everything has been fine for years. For the last couple days one user on the domain has been getting mail rejected back intermittently from one recipient domain. The error is "#5.5.0 smtp;553 #5.1.8 Domain of sender address does not resolve". I've seen a lot of questions with a similar error that the domain does not exist, but this one says the domain does not resolve.
What has made this hard is it might work fine for a half hour and then all of a sudden a message will be rejected again.