Kev, I'm with you. It isn't the people who fail to answer an email that irritate, it's trhe ones who delete AND purge the message without replying. We used to have an email system (Novell) that would let the sender track the status of sent mail, and one particular idiot (yes, digital sociopath is a fitting term) would read, delete, and purge - and yet expected me to be responsive to him!
One of my pet hates at work are people who take ages to get back to you via email, the worst are the ones who have the personal philosophy that "if it's urgent they'll email again" and delete your initial email.