SBS 2003 Outgoing SMTP non-NAT (block of IP) reverse DNS problem

Posted on 2006-03-27
Last Modified: 2010-05-18
Working on an installation with a non NAT ISP connection (block of 10 IP's) is the main IP address is the mail gateway

Domain is hosted remotey and resolves to host IP address OK

MX record for points to for the incoming mail server - receives mail OK

When email is sent out however it sends through the main IP address
If the receiving domain uses revers DNS the IP address does not resolve to the domain / mail server and fails to send

My initial thoughts are to setup two MX records - "outmail" and the other "inmail"
inmail will point to - mail gateway
oumail will point to - main IP address

Then change the SBS virtual server properties with FQDN and Smart Host of

Would this work or is there a simpler / better way of resolving the ougoing mail and reverse DNS resolution problem ?

Thank you for reading my post and considering your reply.
Question by:NiSiWi
    LVL 15

    Expert Comment

    That would work with no problem.
    However you could setup the 2 IP's as in and out you just have to route your SMTP port.
    That will give some kind of redundancy just setup the 2 IP's With priority 10, 9 you got the point.
    LVL 5

    Accepted Solution

    I think just getting your ISP to configure a reverse dns entry for both IP's should work without any hassle.

    The reverse DNS doesnt even have to match to your email domain, there just has to be one there.

    Go to and check out whether you have one or not.


    Author Comment

    This is now very confusing

    I have created two MX records - one for inmail and the other for outmail and both point at IP address .217 and .218.
    When I run the dnsreport everything seems to check out OK, however mail is still being rejected by a specific domain as below.

 on 28/3/2006 21:47
                There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.
                < #5.5.0 smtp;550-Verification failed for <>>

    Any ideas ?
    LVL 5

    Expert Comment

    mx records are not the same as reverse dns - do both have revers dns entries?

    Author Comment

    turns out that I needed to turn off the email for the hosting account and everything started working OK

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