A greylist (also spelled graylist) is a list of e-mail addresses or domain names that a spam filter uses to identify suspected spam. If a message arrives from an address or domain on the list, it will be quarantined and then delivered to the subscriber only if the sender attempts to send the message again within a certain period of time... (Continued)
Sender ID is Microsoft's proposed e-mail sender authentication protocol designed to protect against domain spoofing and phishing exploits.
Vouch By Reference (VBR) is a protocol for adding third-party certification to email. The first part of the protocol requires the sender to add information to their email header by creating a field called VBR-Info.