Keymail.
A keymail is a sort of email. A keymail is a private message between you and another user. The keymails are recorded on the server, so you can send a message to someone who is not connected to the server right now, and the person will receive the message later.

A keymail is not an email: You cannot use someone's email address, and people who don't have a keyja.com nickname can not send a keymail to you. Keymail is internal to the service at keyja.com.

You can block incoming keymails if you don't want to receive them. To do that, use the options in the menu.