When you configure an e-mail client, you typically have to enter the name of your IMAP (or POP-3) server, your SMTP server, etc. Ian points out a new draft standard Use of SRV records for locating email services, which proposes to use DNS SRV resource records to facilitate e-mail client configuration. Imagine launching an e-mail client, specifying your e-mail address, and the rest (bar passwords, of course) happens automatically.
The client uses the "domain" portion as the service domain to perform SRV lookups for the services it wants to configure. If the SRV lookup is successful the host name and port for the service can be determined and used to complete client configuration. If an SRV record is not found, the client will need to prompt the user to enter host name and port information directly.