GTM using WideIP along with DNS zone transfers to ISP

The first part of the test is ActiveSync traffic for mail on mobile devices. The F5 box is providing the external DNS services and the wide IP setup for load balancing / availability. We are also doing a Zone Transfer to our ISP as a backup external DNS server. Public records and our F5 is the SOA. Its my understanding that either our DNS Server or the ISP's DNS server can respond to queries and my thought is that if the ISP DNS responds that the Wide IP would not work correctly?

Once we had things configured my mobile device was able to connect and send / receive mail. Several days later I got a "Cannot Get Mail, The connections to the server failed" my assumption is no response. I am looking for an App for iPhone that maybe able to help figure out the connection problem from the phone side next time it can not connect.

Hopefully this helps, im new to this.