[CLUE-Tech] Sending mail using an IP address?

[Kevin Fenzi]

>>>>> "Jed" == Jed S Baer <thag at frii.com> writes:

Jed> Greetings folks.  Maybe I'm confused, or just remembering simpler
Jed> times.

Jed> Due to some dns resolution problems, I tried sending an e-mail
Jed> directly to a mail server I identified using a dns mtx query,
Jed> i.e. theperson at n.n.n.n - which bounced, because of a host lookup
Jed> failure at my ISP. Now, I can understand how it might bounce at
Jed> the other end, because it might rely on having the domain name
Jed> component for identifying the recipient, if the mailserver is
Jed> handling multiple domains. But I just don't buy the MTA being
Jed> unable to recognize a dotted-quad IP address and using it.

It will always do a lookup on the part after the @ because it needs to
look for MX records. :) 

Jed> I really don't want to become an SMTP guru, or a sendmail or
Jed> postfix guru either. So maybe somebody has nice URL where this is
Jed> explained, or could tell whether my ISP is blowing smoke, or
Jed> whatever. Here's what they said:

Jed> < quoth > All mail is handled by name, not by IP address.  This
Jed> is an internet-wide standard that everyone adheres to.  You
Jed> cannot send email to an IP address only.  I have talked with our
Jed> system administrators about this, and I have done some testing
Jed> myself, sending to different email addresses at the mail server's
Jed> IP address.  I have sent through many different mail servers and
Jed> they all have this same problem.  Mail absolutely has to be
Jed> handled through DNS, so sending directly to the IP, instead of
Jed> the name, is not possible.  < end quoth >

well, they are mostly correct, but not entirely. 

You can address your email to:

username@[n.n.n.n] 

and it will do the right thing. The []'s tell it to not do a lookup,
just send direct to that IP. Not sure if thats part of the standard,
but all the MTA's I know of honor it. 

Jed> But really, isn't sending mail, well using SMTP anyway, just a
Jed> matter of having a server/daemon listing on the proper port, and
Jed> sending a properly formed message to it?

yes, but your outgoing MTA tries to look up the domain/host part of
the address so that it can see if it has an MX record. If it doesn't
then it just sends to the host, but it has to resolve. 

kevin
-- 
Kevin Fenzi
MTS, tummy.com, ltd.
http://www.tummy.com/  KRUD - Kevin's Red Hat Uber Distribution