Sort of defeats the purpose of Posterous though. It's nice to be able to send an email and be done with it. Though for an account coming under constant attack, it'd be nice to have the option though.
Edit: What they should really do is obfuscate the posting email addresses a little. Make my posting email 1234randomwords@posterous.com, and give me the option to change it to something else if I am coming under attack.