Question:

Why is it that when sending Approved Emails to Gmail addresses the emails are dropped, but our internal Domain is receiving emails just fine?

Answer:

This is because Google is using a stricter form of Email Authentication than the internal domain in question and does not accept emails from Spoofed Addresses. An example error message for this situation is:

Unauthenticated email from exampledomain.com is not accepted due to Google's DMARC policy. 
Please contact the administrator of exampledomain.com domain if this was a legitimate mail.

To remedy this, you'll need to register the domain used for sending the emails with the Veeva Mailgun System so that all emails sent on behalf of that domain are properly authenticated. This can be done by creating a support ticket to request that your specific domain is registered.

Related Documentation:

CRM Help Documentation: Creating A Sending Domain