Spiped – secure pipe for SSH, SMTP, etc.
To set up an encrypted and authenticated pipe for sending email between two systems (in the author's case, from many systems around the internet to his central SMTP server, which then relays email to the rest of the world), one might run dd if=/dev/urandom bs=32 count=1 of=keyfile spiped -d -s '[0.0.0.0]:8025' -t '[127.0.0.1]:25' -k keyfile on a server and after copying keyfile to the local system, run spiped -e -s '[127.0.0.1]:25' -t $SERVERNAME:8025 -k keyfile at which point mail delivered