A private message is a message intended for a single recipient.
It is encrypted using the receivers public key. Only the receiver can decrypt the message.
if a private message reaches its destination the receiver sends back an ACK Message to notify the sender of the successfull decryption of the message.
Private messages expire after 2.5 days and are resent in the intervals of power to the number 2: 2 days, 4 days, 8 days, etc. This is repeated until acknowledge is received or the message is deleted from the "sent" tab. Resending a private message forces the client to do POW again.