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SMTP gate problem

Posted on 2003-03-20
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Last Modified: 2008-02-01
Hi,
   i use a routine(C++) to send SMTP mails. When i use it to send mails through the smtp gate (not the POP3 gate) of the ISP that i login the message is send perfectly. When i use a different smtp gate (mail gate from other ISP) most propably if not always it fails. Does the ISP i login blocks this mails to different mail gates? Any idea to overcome this problem?
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Question by:ttsenis
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5 Comments
 
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Expert Comment

by:Hartofiowa
ID: 8174136
Most ISP's now require you to use the SMTP server of the ISP.  You can obviously still use multiple POP3 servers but at least here in Iowa, 95% of the ISP's require the use of their own SMTP server.  I'm even starting to run across ISP's blocking third party POP3 accounts also.
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by:deroode
ID: 8174323
Hartofiowa,

that is not quite true:

The reason ISP's are blocking mail from the SMTP gate is to prevent relaying (and spamming through their servers). But eventually, mail that is destined for users on that particular mailserver is accepted (of course). So when sending mail you have to use either the SMTP server of the ISP that handles your mail, or the SMTP server of the ISP that holds the user mailbox the mail is destined for. That is by the way exactly how all SMTP mail server work: find out which mail server to send the mail to (through the DNS MX record), and then send it to the SMTP server of that domain.
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by:Hartofiowa
ID: 8174490
Deroode,

I just want clarification on something you stated.  Yes, you are correct that you can also use the SMTP server of the ISP that handles your e-mail, along with the SMTP server of the ISP providing your internet connection.  The thing that doesn't make sense to me is your statement that you can use the SMTP server of the ISP holding the mailbox account of the user you are trying to send the e-mail to.  

I just tried that as a test and it didn't work for me.  Here is my setup.  I changed the SMTP server in my Outlook account to both my ISP's SMTP server and the SMTP server of my companies mailhost.  I can send e-mail successfully to my dialup access e-mail account with both settings.  But when I use the SMTP server of the third party e-mail account it doesn't work.  Am I misunderstanding something you said or is there a configuration that would allow me to do this also?
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Accepted Solution

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deroode earned 375 total points
ID: 8179827
Hartofiowa,

Have you checked the MX record of the 3rd party e-mail account? Most often the SMTP server that is used by all users of an ISP to send mail out isn't the same that is advertised in the MX record to receive mail for the domain.

Example:
My dutch provider 12move.nl, helpdesk page, mail settings:
SMTP server: smtp.12move.nl, IP 195.241.76.179

MX records for 12move.nl:
12move.nl       MX preference = 300, mail exchanger = relay3.tiscali.nl
12move.nl       MX preference = 100, mail exchanger = smtpcp.12move.nl

with different IP addresses. smtp.12move.nl accepts only mail from the 12move.nl IP range, smtpcp and relay3 accept only mail directed to the 12move.nl domain

Hope that helps..

b.t.w. How to see the MX records:
nslookup
type=MX
12move.nl
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by:CleanupPing
ID: 9153001
ttsenis:
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