Exchange ActiveSync push mail not working over VPN

Hello,

we setup various VPN types (AnyConnect, PPTP, IPSec) to connect our iOS devices to our corporate network and let users access their Microsoft Exchange mails. Everything except for the push notifications is working fine. Even though the VPN tunnel is up and running (the device can be pinged) no new mails get pushed to the device, the user needs to manually refresh the Inbox to receive new mails.

Under "Settings" everything set to use "push", this also goes for the Exchange specific settings under "Advanced".

If we connect the iPhone directly to the Exchange system without the VPN, "push" mails are working properly.

The Exchange server logs show no indication that the iPhone is issuing the "CMD=Ping" when connected through the VPN.

I could find no statement neither by Apple, Microsoft or CISCO stating that "push" should not work through VPN on the iPhone.

Can anybody help me find something official or documented in this regard?

Best regards

RB