Tom Murphy notes the difficulty PR practitioners have in knowing their messages are getting through the proliferation of spam.
One quick way to check whether your press release will probably get through, is to use a free online service called spamcheck:
Send your press release to: firstname.lastname@example.org
Put the word "TEST" at the beginning of the subject line and you'll get an automated response in a couple of minutes rating the "spam-content" of your press release!
In a previous post Murphy also mentions when sending a journalist a release he puts PR in the subject line, so as to be very clear where the message is coming from. I agree that there's little point in trying to pretend your message is something it isn't, and announcing a press release in the subject line will likely help in pass through the spam blockers.