I hope this is an appropriate question for here. I'm a software guy, I usually do "ok" figuring stuff out, but I can't get this one. I know it's easy, too.. I just can't get it, and I need help.

Problem arose after switching to Comcast Business.

  • Comcast Business router, Cisco DPC3941B
  • Static IP, DMZ to server
  • SmarterMail Email Server

Email is not sent from, nor delivered to phones on local network wifi. If I turn wifi off on the device, and "resend" over 4g, success. If I am connected to any remove wifi network, success. Issue is only over local network.

I have no idea if this is a setting in SmarterMail, in my router, or what to do. I'm hoping that you guys can have mercy and give me a hand.

Please let me know what information you need if you are willing to assist. I don't even know what router or server settings to give you.

Thanks in advance guys, R/

NAT Configuration 1 [static IP address] [Enabled]

No Static Routing

Dynamic DNS Disabled

Let me know if this is enough or if anything else is needed.

  • 1
    The config for the Cisco router would be helpful. My guess is it has something to do with NAT configured on the router.
    – Ron Trunk
    Feb 25 '17 at 15:28

With only the info provided, it sounds like the issue is that the devices, when connected to the local WiFi, are attempting to contact the public IP address used to connect to the mail server.

When a connection to the mail server is made from inside the same network as the server, the traffic is attempting to "hairpin" through the router.

Client -> private IP on near ("local") interface of router -> public IP on far ("Internet") interface of router -> private IP on near ("local") interface of router -> Server

Same diagram, rearranged to demonstrate the hairpin:

Client -> Router (local) -> Router (public) -
Server <- Router (local) <-------------------

When the same connection is attempted from another network, the hairpin is not present:

Client -> Internet -> Router (public) -> Router (local) -> Server

The local interface on the router is not traversed multiple times for connections coming in from the outside.

There are lots of problems associated with hairpinning traffic through network devices.

If your email client supports multiple server profiles, you could set up a profile that will work internally and one that will work externally.

A better solution would be to have the hostname used in the email clients to resolve the internal IP address when on the internal network and resolve to the public IP address elsewhere. Make sure to keep the TTL of the DNS record low enough that it will not be cached for long to minimize the delay when transitioning between internal and external networks.

Wikipedia entry on Hairpinning

-Rich Alloway (RogueWave)

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