SMTP and Sendmail
The Simple Mail Transfer Protocol (SMTP) is responsible for transferring email messages from one server to the next. It's a simple protocol, as its name suggests, designed only for establishing the identity of the sender and the recipient, and transmitting a message's contents. The sender and recipient are each specified by an email address, which consists of a username and a hostname, separated by the at symbol, @; if the SMTP server that receives the message has the same hostname as the one in the email address, it knows the message is intended to reach a recipient there. If not, it knows it must relay the message to another host.
In the simplest case, this is the way an email message travels from one user to another; in the ...
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