electronic postmaster
An electronic postmaster is the capability in a program, usually a special program designated as an e-mail server, for handling the distribution, forwarding, and receiving of e-mail in a network. For example, an Simple Mail Transfer Protocol server at your access provider acts as an electronic postmaster by forwarding your outgoing messages to their destinations in the network and by collecting your incoming messages so that you can request them from a POP3 server that holds your messages until you request that they be sent to your workstation client.
When you send a message with an e-mail address that can't be located, the term "electronic postmaster" may appear in the message sent to you by the SMTP server.