This is a potential security issue, you are being redirected to https://csrc.nist.rip.
A host that provides “electronic post office” facilities. It stores incoming mail for distribution to users and forwards outgoing mail. The term may refer to just the application that performs this service, which can reside on a machine with other services, but for this document the term refers to the entire host including the mail server application, the host operating system and the supporting hardware.
NIST SP 800-45 Version 2