firstname.lastname@example.org sends email to email@example.com. Bob is on a different network and authenticates over SSL to send his email. Both are in the same domain, yet the message is checked for spam. I don't care what this email is, but I want it to get through. Mail sent through the Zimbra web UI to another local account is not checked for spam -- why should this be?
To sum this up, I am ok with email leaving the domain being checked for spam. However, I think that mail between users in the same domain should never be checked if the user has authenticated themselves.