Postmaster

On the DH panel, while managing your email addresses, you may see this message:

You should always have valid postmaster@domain.com and abuse@domain.com addresses, or some domains may not accept email from you.

If you're wondering why this might be true, see http://www.rfc-ignorant.org/ - a service that allows users to submit domains that don't have a "postmaster" and/or "abuse" address, and that also allows mail servers to automatically refuse email from domains that were blacklisted in this way.