E-mails are distributed using a (OSI Layer 7) protocol called Simple Mail
Transfer Protocol (SMTP). SMTP normally operates on powerful computers
dedicated to e-mail distribution, called SMTP servers. When you create and
send an e-mail, your e-mail client sends the file to the SMTP server. The server
pulls out the addresses from the message. (You can send e-mails to multiple
recipients.) For each domain name, the SMTP server must send a message to a
DNS to get the IP address of each recipient’s e-mail server. If the recipient is on
the same server as you (that is, if you send an e-mail to someone with the
same domain name), this step is unnecessary.
After your SMTP server knows the IP address of the recipient’s server, your
SMTP server transfers the e-mail message to the recipient’s SMTP server. If
there are multiple recipients in different e-mail domains, a separate copy of the
e-mail is transferred to each recipient’s SMTP server. According to the name of
the protocol, this is all pretty simple.