nncp-daemon can be connected with nncp-caller for a long time – it can create TCP connection that lasts for many hours. When SMTP server receives mail, it will call nncp-exec creating an outbound encrypted packet. Daemon checks outbound directory each second and immediately sends notification about undelivered packets to remote side, that also downloads it at once.
There are only dozens of bytes notifying about incoming packets, dozens of bytes telling to download those packets. Mail packets are compressed (POP3 and IMAP4 as a rule do not). You have lightweight, compressed, low-delay, reliable link for the mail with strong encryption and mutual sides authentication!