An entity that takes an email message created by the sender’s mail user agent, and delivers it to the receiver’s mail user agent. If delivery is not possible because the receiver’s mail user agent is not active (for example because it runs on a personal computer that is currently disconnected from the network or is not powered on), the message will be placed in a message store to await collection by the receiver’s mail user agent when it becomes active. A computer dedicated to running MTAs is called a mail server.