Guide to Unix/Explanations/Mail Server Overview/Mail Delivery Agent
The mail delivery agent software distributes mail messages to recipients individual mailboxes.
The mail delivery agent is generally not started from the command line, but is usually invoked by a mail delivery subsystem component, such as a mail transport agent, or a mail retrieval agent.
Explicit Delivery Mode
Explicit Delivery Mode is an operation mode supported by some mail delivery agent software packages. A mail delivery agent operating in explicit delivery mode is capable of delivering mail to local user recipients. The local recipient's username is passed as a command line parameter to the mail delivery agent. In order to operate in such a mode, the mail delivery agent needs to be configured to run with sufficient privileges to be able to deliver mail to the local recipients mailbox.