The Spam Reduction tab lets you configure various ways VPOP3 can reduce spam and email address spoofing
“Domainkeys” is a way of signing a message to show that it comes from the person it claims to come from.
The system is too complex to go into here, but it essentially takes key parts of the message header, as well as the message body and creates a PKI digital signature on it, using a 'private key' on your system. The receiving end will find the same message header parts & body and verify the digital signature using a 'public key' which is distributed using the DNS system.
If you have VPOP3 signing outgoing mail using Domainkeys, you should store the private keys in PEM format in a file called domainkey_<domain name>_<selector>.key in the VPOP3 directory. For instance domainkey_pscs.co.uk_1.key
VPOP3 will automatically choose the appropriate private key file depending on the sender's email domain.
One disadvantage of Domainkeys happens if some other software alters the message content. For instance, if something adds a 'this message has been checked for viruses', or a company signature, then that will alter the message content so that the domainkey signature will not be verified.
For more information on Domainkeys, you can see: http://www.unlocktheinbox.com/resources/domainkeys/