An ambiguity in RFC 2821, which defines how email should be delivered, causes problems for some users, according to Ferris Research. In their first blog on the subject they relate a story of someone (names are expunged to protect the innocent from embarrassment) who decided to configure his DNS with both an MX record (which advertises the mail server) and a CNAME record defining where the web server was. More specifically, the CNAME defined "the-domain-in-question.com." to be "www.the-domain-in-question.com", the IP address of which was defined in a separate A record. After this, Mr. Anonymous's e-mail wasn't consistently reaching the mail server anymore. Some external servers were no longer finding the mail server. The problem turns out to be that when a server has a CNAME record some sending mail servers will attempt to connect to that and not to the server pointed to by the MX record. So in the example, the outside mail was being sent to the web server, which of course didn't respond to it. The problem, says Ferris, is in an ambiguity in RFC 2821. They have a point. The SMTP standard seems to recommend against mixing CNAME and MX records, but it doesn't prohibit it, and it's unclear on how the server should behave when it finds both. Bottom line: Don't mix them.