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MXRecords and reverse lookup for SMTP services.

Posted on 2008-10-17
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Last Modified: 2013-12-14
Greetings EE Gods.....

I have an exchange server that sends SMTP out one IP, however gets NAT'd to a second.  In Addition, the outbound IP address is NOT MXRecorded because that is not our Primary IP address to the ISP.  I have three, and what I need to do is fool the destination end to believe it is coming from one IP vice another.  My outbound data is *.*.*.5, however my MXRecord is on my primary line at *.*.*.3

I need the data to continue out on .5, but have the destination think its coming from .3 so I don't get caught in spam filters.  I do NOT have the ability to add a record to the MXRecord.

Thankyou.
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Question by:mtmadhatt
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by:Chris Dent
ID: 22745703

The MX Record effects Inbound mail only. It has no impact at all on Outbound mail.

On the other hand, you must have a PTR record and a valid name for the IP address the server does use to send out mail. Can you add those? The name will be via your DNS host (if that's not yourself), and the name must be set on the SMTP service. The PTR record is normally set by your ISP (or whoever provides the network connection the SMTP server uses).

If you can't have those then you must relay mail out through a third party if you expect mail to be delivered reliably.

Chris
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by:mtmadhatt
ID: 22745789
ok. We know that our PTR records point to *.*.*.3, but our data goes out *.*.*.5.
We are having difficulty getting our PTR, sorry for the  bad nouns, moved.

How can I virtually make the distant end think *.3 even though it is coming from *.5?

Thanks.
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Chris Dent earned 500 total points
ID: 22745831

No, I'm afraid not.

You need to be able to establish a TCP connection, that means it needs to get the response back from the remote server. It cannot possibly get that response if you're faking the IP (bad return path).

You can't change the network address translation so it uses .5?

For the PTR records, it's basically an IP pointing to a name, so rather than it pointing to .3, it's .3 pointing to a name (if you see what I mean).

Chris
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Author Closing Comment

by:mtmadhatt
ID: 31507339
I was afraid you were going to say that.  Ok, so it sounds like i"m down to two possible solutions.  I know where to go from here.
Thanks.
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Expert Comment

by:Chris Dent
ID: 22745911

Good luck :)

Chris
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