[techtalk] POP mail

Laurel Fan lf25+ at andrew.cmu.edu
Wed Dec 15 18:58:39 EST 1999

Excerpts from linuxchix: 15-Dec-99 RE: [techtalk] POP mail by Di
Gregory at sleepy.giant. 
> The thing that really got me, is that when I hooked up my old roommate's 
> email through POP, the school's servers will send it if you're not dialed 
> up into them, but when he sent an email, the receiver's server somehow 
> sensed that he was relaying it from another domain, through the school's 
> SMTP server.

POP uses a password to authenticate, so it doesnt care what domain
you're from, as long as you have the correct login and password.

A) To send mail to a person, you connect to the smtp server on their
mail host. For example, to email me, you connect to the smtp server on
andrew.cmu.edu[1], because the server knows the recipient.

B) If you want to, you can also use a gateway, and tell your smtp server
to send it to the recipient's for you.  I could connect to the smtp
server on andrew.cmu.edu, and it'll send to anyone,[2] because the
server knows the sender.

C) However, the server can decide whether it wants to be used as a
gateway.  Most servers only allow those in the same domain to do so.  It
is called relaying when neither the sender or the recipient is known to
the server, and many servers don't allow this, mostly because of

So, basically, you need to find a smtp server that knows you.  For
example, to find the mail server for rr.com, I did:

  dig rr.com mx

I could also do:

  host -t mx rr.com

(mx is the query type, and stands for Mail eXchanger) [4]

Which gave me:
rr.com                  MX      10 acme-reston.va.rr.com
which tells me that acme-reston.va.rr.com is the mail host for rr.com. 
You can use this method to find mail hosts for your domains.

[1] For example, my mail host is andrew.cmu.edu.  So, to send mail to me:
% telnet andrew.cmu.edu 25
Connected to andrew.cmu.edu.
Escape character is '^]'.
220-andrew.cmu.edu ESMTP Sendmail 8.9.3/8.9.3
220-Mis-identifying the sender of mail is an abuse of computing facilites.
220 ESMTP spoken here
HELO annex-9.ece.cmu.edu
250 andrew.cmu.edu Hello ANNEX-9.ECE.CMU.EDU [], pleased to meet you
MAIL from: lf25 at andrew.cmu.edu
250 lf25 at andrew.cmu.edu... Sender ok
RCPT to: lf25 at andrew.cmu.edu
250 lf25 at andrew.cmu.edu... Recipient ok
354 Enter mail, end with "." on a line by itself
This is a mail.

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