A mail server is
|“||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, or it can refer to the entire host including the mail server application, the host operating system and the supporting hardware.||”|
- NIST Special Publication 800-45, Glossary, at A-1.