or Qmail.
SMTP allows each computer that the email passes through to forward it in the
right direction toward the final destination. When you consider the millions of
email servers around the world, you have to marvel at how simple it all
seems.
Here is a simplified example of how email is successfully processed and sent
to its destination:
- [email protected] composes and sends an email message
to [email protected]. - The MTA at seymourcray.net receives Matthew’s email message
and queues it for delivery behind any other messages that are also waiting
to go out. - The MTA at seymourcray.net contacts the MTA at
gracehopper.net on port 24. After gracehopper.net
acknowledges the connection, the MTA at seymourcray.net sends
the mail message. After gracehopper.net accepts and acknowledges
receipt of the message, the connection is closed. - The MTA at gracehopper.net places the mail message into Sandra’s
incoming mailbox; Sandra is notified that she has new mail the next time
she logs on.
However, several things can go wrong during this process. Here are a few
examples:
What if Sandra does not exist at gracehopper.net? In this case, the
MTA at gracehopper.net rejects the email and notifies the MTA at
seymourcray.net about the problem. The MTA at
seymourcray.net then generates an email message and sends it to
[email protected], informing him that no Sandra exists
at gracehopper.net (or perhaps just silently discards the message
and gives the sender no indication of the problem, depending on how the
email server is configured).
What happens if gracehopper.net doesn’t respond to
seymourcray.net’s connection attempts? (Perhaps the server is
down for maintenance.) The MTA at seymourcray.net notifies the
sender that the initial delivery attempt has failed. Further attempts will
be made at intervals decided by the server administrator until the
deadline is reached, and the sender will be notified that the mail is