SMTP Overview

temporary failures (but not too soon or too often) and giving up gracefully on permanent failures. (Tragically,
there are improper clients out there in the world.)
A SMTP conversation between the client and the server goes in stages, each one initiated by the client doing
something. A typical conversation will look like:
Client does:
Connects to the server
220 Helo there
Server normally responds with:
HELO client-hostname
250 Pleased to meet you
MAIL FROM:
250 OK
RCPT TO:
(May be repeated)
250 OK
DATA
354 Start mail input; end with .
Sends the actual email message
(Nothing, it's waiting for the . that ends the message)
. 250 OK, accepted for delivery
At this point the email message has been sent. The client can now disconnect with a QUIT command, or it can
send another email message by starting with the MAIL FROM step again (optionally sending a RSET command
first).
The sender address is the email address that will receive email about delivery problems (mailing lists change
this but not the From: email header so that they, and not the people sending to them, get messages about
delivery problems). A special null sender address (MAIL FROM:) is used to signal that no one cares and no
bounce notifications should be sent. Null senders are used when sending bounce messages themselves, and
sometimes at other times.
There can be multiple recipients of the same message on the same computer. So that the actual email message
only has to be transfered once (saving bandwidth), there can be several RCPT TOs for a message. (There has to
be at least one, just like there has to be a MAIL FROM.) The client has to keep track of which recipient addresses
have problems, if any, and retry them later if necessary.
The envelope headers are the MAIL FROM and RCPT TO parts of the SMTP conversation. The envelope sender is
the MAIL FROM address, and the envelope recipients are the RCPT TO addresses.
The client-hostname, the sender address, and the recipient addresses should all be fully qualified. A fully
qualified host or domain name is one that anyone on the Internet could use to look up information, not a
shortened name useful only on machines inside an organization; for example, server.example.com instead of
just server. A fully qualified email address is an email address with a fully qualified host or domain name, not
just an email login; for example, MAIL FROM: instead of MAIL
FROM: or MAIL FROM:. If the host or domain name is left off an email
address, the SMTP server usually has no choice but to interpret it as an address on itself.