A program responsible for receiving incoming messages and routing the messages to individual users.
A specialized program for routing and delivering messages. MTAs work together to transfer messages and deliver them to the intended recipient. The MTA determines whether a message is delivered to the local message store or routed to another MTA for remote delivery.
An e-mail delivery program that accepts a message from either a MUA or another MTA. The message is stored locally while the MTA determines the message's destination and delivery method.