SMTP Not Showing Correct Connecting IP

Posted on 2011-04-25
Last Modified: 2013-11-30
I am sending mail using MS SMTP on a IIS5 server. The physical address of the server is However, when a receiving mail server gets the mail, it recognizes the Connecting IP as, which is my gateway IP.

How do I get receiving mail servers to recognize the Connecting IP of the physical server, rather than the gateway IP?

Thanks in advance for any assistance.
Question by:toddheinz2
    LVL 57

    Expert Comment

    Is the IP address physically configured on your SMTP server?

    Or is there some device (say your gateway) supposed to be doing NAT?

    It sounds like your gateway is supposed to be doing NAT, but is not.

    Author Comment

    Hi giltjr - thanks for the response!

    Regarding the SMTP configuration. In IIS5, the only IP config field that I see is on the PROPERTIES / GENERAL  Tab (see image below). It calls out to the server private address:

     SMTP Properties Screen Capture
    We have a Sonicwall firewall and I am confident the NAT configuration translates the private IP of to a private IP of

    Any more thoughts?
    LVL 57

    Accepted Solution

    There are only two possibilites that I can think of:

    1) The SMTP server is not using as the source address when sending e-mail.

    2) The NAT on the firewall is not setup correctly.

    #1 is easily checked by installing a packet capture utility (either MS NetMon or Wireshark) and then running a packet capture and see what the source IP address is.  If it is, then the firewall is not setup properly.

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