When configuring a properly set up mail server, there are a number of DNS records you need to set up in order to ensure that the system functions as expected. Some of these are defined in RFC974, which covers mail routing and the DNS system.
The main piece you need to understand is
Mail Exchanger (MX) records. These records define how e-mail destined to a given domain should be handled. Without an MX record being defined, e-mail addressed to user@domain.com would be sent to the domain.com A record, which is often an HTTP server. For some smaller sites, this may be reasonable if their HTTP server is also an SMTP server, but that is not always the case.
Rather than depending on the A record, you can instead use one or more MX records with defined priorities that point to A records which may be in or out of the domain you're configuring. For example, your e-mail could go to smtp.domain.com. Alternatively, if Google Apps handles your e-mail, you may...
