Analysis of information sources in references of the Wikipedia article "Email client" in English language version.
In addition to providing remote shell access and command execution, OpenSSH can forward arbitrary TCP ports to the other end of your connection. This can be very handy for protecting email, web, or any other traffic you need to keep private (at least, all the way to the other end of the tunnel).
ssh accomplishes local forwarding by binding to a local port, performing encryption, sending the encrypted data to the remote end of the ssh connection, then decrypting it and sending it to the remote host and port you specify. Start an ssh tunnel with the -L switch (short for Local):root@laptop:~# ssh -f -N -L110:mailhost:110 -l user mailhost
Naturally, substitute user with your username, and mailhost with your mail server's name or IP address. Note that you will have to be root on the laptop for this example since you'll be binding to a privileged port (110, the POP port). You should also disable any locally running POP daemon (look in /etc/inetd.conf) or it will get in the way.
Now to encrypt all of your POP traffic, configure your mail client to connect to localhost port 110. It will happily talk to mailhost as if it were connected directly, except that the entire conversation will be encrypted.
APOP, replaces the standard USER/PASS authentication with a challenge-response authentication mechanism. This solves the problem of the disclosure of reusable passwords, but does nothing to prevent eavesdroppers from reading users' mail messages as they're being retrieved." Sill, Dave (2003). The qmail Handbook. Apress. ISBN 9781430211341.This document does not provide recommendations on specific security implementations. It simply provides a warning that transmitting user credentials in clear text over insecure networks SHOULD be avoided in all scenarios as this could allow attackers to listen for this traffic and steal account data. In these cases, it is strongly suggested that an appropriate security technology MUST be used.
Some of this information has previously been sent in non-standardized header fields such as X-Newsreader, X-Mailer, X-Posting-Agent, X-Http-User-Agent, and others
Headers defined only in RFC 1036 for use in Usenet News sometimes appear in mail messages, either because the messages have been gatewayed from Usenet News to e-mail, or because the messages were written in combined clients supporting both e-mail and Usenet News in the same client. These headers are not standardized for use in Internet e-mail and should be handled with caution by e-mail agents.
This document does not provide recommendations on specific security implementations. It simply provides a warning that transmitting user credentials in clear text over insecure networks SHOULD be avoided in all scenarios as this could allow attackers to listen for this traffic and steal account data. In these cases, it is strongly suggested that an appropriate security technology MUST be used.
Some of this information has previously been sent in non-standardized header fields such as X-Newsreader, X-Mailer, X-Posting-Agent, X-Http-User-Agent, and others
Headers defined only in RFC 1036 for use in Usenet News sometimes appear in mail messages, either because the messages have been gatewayed from Usenet News to e-mail, or because the messages were written in combined clients supporting both e-mail and Usenet News in the same client. These headers are not standardized for use in Internet e-mail and should be handled with caution by e-mail agents.