Email Permission Keys, or Message Keys are just a way of embedding a code in to an email address in such a way that the code is persevered during normal use.

There are three different types of base keys, and numerous types of hybrid keys.

Typical keys may look like this:

"John Smith -12345"
"John Smith ( This e-mail address is being protected from spambots. You need JavaScript enabled to view it )" This e-mail address is being protected from spambots. You need JavaScript enabled to view it
john.smith+ This e-mail address is being protected from spambots. You need JavaScript enabled to view it
joHN.SmiTH+ This e-mail address is being protected from spambots. You need JavaScript enabled to view it
This e-mail address is being protected from spambots. You need JavaScript enabled to view it

Message Keys are:

• a unique code or key that is embedded in to an email address
• embedded in such a way that it is likely that the code be retained during normal use of that address
• compatible with the existing Internet infrastructure
• an autominius requires no modification to existing third party processes

They are not domain keys, they are not SPF (Sender Policy Framework) enhancements. They do not require the cooperation of third party postmasters or modifications to the existing internet infrastructure. They don't need to be present in every message that you receive - as messages without keys or with invalid keys, are handled as they were prior to the introduction of the message keys system, and so are therefore no worse off.