From Gate Comm:
PostCast Server Professional completely replaces your ISP's SMTP server. By making just one change in your e-mail program, you are able to send messages directly from your computer to the recipient of every message you send. Every message sent through your ISP server could be read by any person who has access to their computer. Any administrator or employee or any busybody can read your messages. If you are sending confidential information, like your credit card data and the like, this might create a problem.