MailPGP was a frontend for the old command-line versions of Pretty Good Privacy (PGP). The development of MailPGP was stopped soon after Pretty Good Privacy 5.0 with a built-in graphical user interface was released.

MailPGP is no longer available for download at this site. If you absolutely want to try it, you may be able to find it somewhere else. However, please remember that MailPGP is not supported by the author and its development will not be continued.

I recommend The GNU Privacy Guard with WinPT frontend for Windows, and KGPG frontend for Linux (and other platforms).