Earlier this week, Microsoft said open-source programs step on 235 Microsoft patents. The core of the free Linux operating system violates 42 patents. Open-source programs' graphical user interfaces, or the way menus and windows look on the screen, breach 65. E-mail programs step on 15, and other programs violate 68 patents, the company said. The patent figures were first reported by Fortune magazine.
Emphasis mine. So, don't write any SMTP or POP3 clients. You might get sued.