A server that manages and maintains emails sent to and from an end user.
The server authenticates a user by username and password to ensure the privacy of the messages.
It also handles failures to send a message to another receiving end user. If Alice’s server cannot deliver a message to Bob’s server, then Alice’s server holds the message in a message queue and attempts to transfer it later. This is because SMTP normally does not use any intermediary mail servers, that it could otherwise save the message in.