Hi Bullseye,
I'm confused about your question. I'd like to ask the following questions to help us understand your needs:
1. What is your Exchange version?
2. What do you mean about using Gmail ids/SMTP in your domain?
Moreover, the following article may give your some hints:
SMTP relay: Route outgoing non-Gmail messages through Google
Best Regards,
Niko Cheng
TechNet Community Support
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