I basically do this too, though I just put in some random gibberish (or "admin" or "info" or something) before the domain. I figure they probably have some catchall email address, and if not, nothing wasted

On the other hand, if you do want a one-time piece of email, but don't want to be subscribed to a mailing list, check out sharklasers.com. It's a free temporary email service that works pretty well