X-Ray is an email header filter and POP/SMTP server switching tool. It runs as a local POP/SMTP relay server and scans your incoming and/or outgoing mail for user specified mail headers. Once it finds a match it can change the header, remove it, replace it or perform some other special modifications. In addition, the program allows you to set up multiple profiles for POP and SMTP servers and switch between them from the system tray. X-Ray can also run as an NT service.
Any Win32-compatible OS