Email is fast, convenient, informal, easy, and becoming universal.
Companies accept that some private or domestic Email traffic is a natural part of business life. There are copies of every message you send scattered about the Internet, on backup files, and in temporary files, and miss-routed to strange addresses. In many countries a court of law may request copies of Email messages from a companies archive files. Professional firms may wish to include disclaimer on all Email messages in case the Email falls into the wrong hands. I think this looks awful, but the legal department must have their say.