I got the same problem, but fixed by adding Ip of eth1(external) to DNS-Config.
I have checked DNS-Domain i use by openchange->setup->OutgoingMailDomain->"exampledomain.lan"
So i go to DNS-Module, search the right domain, because i have more than one. "example domain.lan".
In this line click on Hosts-Configuration-Button, where i get a list of all my hosts in my domain.
There i search for my openchange server name: in my case: "exchange".
I click on Ip-Adrress-Configuration-Button and then add the IP-Address of my Nic which has "External-Option" activated.
My problem was, i have added the second device after setup dns. So it was not auto-configured.
Hope that helps some people with that problem.
P.S: To activate ActiveSync to get in sync with your mobile Devices: sudo apt-get install sogo-activesync.
Then you get a checkbox for active sync in Openchange-Setup section :-)