GTM iRule when record not found in Zone

I'm pretty new to the BIG-IP DNS with very little DNS knowledge and would like to seek assistance on a challenge that I'm facing.

 

We have an internal GTM with a Master server zone named "company.com" and an A record for "a.company.com" (which is hosted within the company server). The development team has setup another web server "b.company.com" & "mail.company.com" (which are hosted externally) and its A records are managed by the hosting company. We have enabled recursive forwarding to 8.8.8.8.

 

Our end users are able to resolve "a.company.com" from the internal network but is unable to do so with with "b.company.com" & "mail.company.com". My understanding is that the query failed because the A records are not configured in the zone to resolve to.

 

Besides using a CNAME or MX record to point to those external servers, is there a way we can configure an iRule or a script so that the DNS query will query Google NS if it fails to find the host in the zone before sending a failed response? And can we configure it for only this particular zone? The reason for this query is because the development team wishes to host a large amount of web servers externally and a handful internally.