[lug] Sendmail

[John Hernandez]

Greg, I'm going to make an attempt to clarify, because I strongly suspect that you have only a few possibilities:

1) Use your ISP's mail relay (probably the easiest solution)
2) Convince them to drop their smtp filter (or make a special case for you to delivery directly)
3) Refrain from sending e-mail outside your domain
4) Change ISP's

Mail is sent to (and likewise received by) MTA's (such as sendmail) listening on TCP port 25.  If you want to send a message to a domain outside of your administrative control, you need to send to port 25.  To alter this, you would need to pre-arrange delivery to a non-standard port with EVERY destination -- not really practical.  If you ISP does not permit you to directly send to remote SMTP servers, you're pretty much out out of gas and forced to choose between 1-4 above.

There's a subtle issue with the statement you make: "Port 25 for sending and port 110 for receiving."  This is not technically correct and a common point of confusion.  More acurately, port 25 is used for mail relay/forwarding/delivery.  Port 110 is used for mail retrieval by a desktop client.  They are two totally independent functions.  In the former operation, a random high number above 1023 is the source port, and 25 is the destination port.  MUA's (aka "clients", Eudora, Outlook, etc) generally use the POP3 (server listens on TCP 110) or IMAP (server listens on TCP 143) protocols to retrieve messages from your mailbox on a remote server.  These protocols are not involved in the delivery of mail in any way -- think of them as "last mile" protocols that are designed to make it easier to read mail remotely (versus: telnet server;cat /var/spool/mail/user)

Greg Horne wrote:
> 
> "Maybe I'm just missing the entire problem, but you are kind of
> required to use port 25.  That's the port that servers speaking smtp
> traditionally listen and speak on and trying to connect to someone else
> on port 2500 won't work unless the other end is configured to use 2500 as
> well."
> 
> Yeah, assuming it could be done I would configure the my smtp server AND my
> clients to both use a different port that is not being blocked by the ISP.
> 
> Greg
> 
> >From: Chip Atkinson <catkinson at circadence.com>
> >Reply-To: lug at lug.boulder.co.us
> >To: lug at lug.boulder.co.us
> >Subject: Re: [lug] Sendmail
> >Date: Fri, 14 Sep 2001 13:51:19 -0600
> >
> >Maybe I'm just missing the entire problem, but you are kind of required
> >to use port 25.  That's the port that servers speaking smtp
> >traditionally listen and speak on and trying to connect to someone else
> >on port 2500 won't work unless the other end is configured to use 2500
> >as well.  If that is the case, then you could probably talk them into
> >relaying mail for you, since they are reconfiguring their sendmail to
> >talk with you anyway.
> >
> >Port 110 is for pop3, which is good for downloading, but that would be
> >used by the desktop machines to download mail from a main server.
> >
> >Can you not set the "Smart" relay host in sendmail.cf to be
> >smtp.ix.netcom.com and restart sendmail?  Then all outgoing email is
> >sent through netcom but should make it to the ultimate destination.
> >
> >The main machine that you configure to use as a relay will then be the
> >main one that the desktop users connect to via port 110.
> >
> >If I am indeed missing something and you can get away with using port
> >2500, you might wish to look into ip port forwarding.  There's a brief
> >how to on it and it would save you from having to tear into the
> >internals of sendmail.  "Merely" use the forwarding to forward
> >connections to and from port 25 to port 2500.
> >
> >Chip
> >
> >Greg Horne wrote:
> >
> >>I would like to use a different port (other than 25) to send mail on.
> >>Since Netcom is blocking 25, i'll use 2500, or some random unassigned
> >>port.  My question was this:  How do I set sendmail up to allow both
> >>port 25, and 2500 to be used to send mail?
> >>
> >>Greg
> >>
> >>P.S.  I am considering this because Netcom was of no help on the phone.
> >>They just said that they only allow their @ix.netcom.com mail to go
> >>through their servers.  I would like to use a relay, but I don't think
> >>that is going to happen.  This port thing is my alternative.
> >>
> >>>From: Chip Atkinson <catkinson at circadence.com>
> >>>Reply-To: lug at lug.boulder.co.us
> >>>To: lug at lug.boulder.co.us
> >>>Subject: Re: [lug] Sendmail
> >>>Date: Fri, 14 Sep 2001 12:13:52 -0600
> >>>
> >>>I'm not sure I entirely understand the problem, but it sounds like what
> >>>Qwest does too.  Basically you have to use their machines as relays for
> >>>outbound mail. Why is this a problem?  I run a moderately busy mailing
> >>>list and haven't had any problem with the relay part.  Originally I
> >>>wanted to do my own delivery to reduce their load, but since they want
> >>>it that way, I just said to myself "screw 'em".
> >>>
> >>>Greg Horne wrote:
> >>>
> >>>>Check this out!
> >>>>
> >>>>So I go on a consulting job to a company to set them up with e-mail
> >>>>through my company.  There e-mail is @company.com, so smtp.company.com
> >>>>is what they would use for mail servers.  Port 25 for sending and port
> >>>>110 for receiving.  I set everybody up, and checking mail was just
> >>>>fine.  When I went to send a test mail they got the error 10,060, which
> >>>>when I looked it up meant that their ISP (Netcom) is blocking all
> >>>>outgoing port 25 communication unless is goes through their mail servers
> >>>>first (smtp.ix.netcom.com).  I sent an e-mail to netcom and am waiting
> >>>>for a response.  I also looked around the net and found that to get
> >>>>around this people are using alternate ports for sending like 2500.  My
> >>>>question is this:  How can I set up sendmail to use port 25 for e-mail,
> >>>>but also use a new port 2500 for the mail @company.com?  Can these two
> >>>>ports work in tandem or would I have to move all clients to 2500?
> >>>>
> >>>>Thanks,
> >>>>Greg Horne
> >>>>
> >>>>P.S.  A lot Netcom does this blocking of port 25 to other smtp servers
> >>>>to prevent their users from spamming.  When a coworker of mine talked to
> >>>>them on the phone they said it was in the terms of use.  Bleh!
> >>>>

-- 

  - John Hernandez - Network Engineer - 303-497-6392 -
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