So who is going to teach Gladys from accounting how to store her contacts' PGP keys and encrypt her email?
Maybe PGP is a little on the difficult side, but at my company, we use a dedicated server for any email that needs to be encrypted. It has a little web app (written by a former employee) people can use to send and receive messages, with attachments, if necessary. All the data is transferred through HTTPS. I don't use it, myself, but it must be pretty easy, because we have to follow HIPAA regulations and we have plenty of people here who aren't exactly computer experts.