No easy answer here. Individual MTAs or a cluster of them typically live under one unique domain. In your scenario, you'd have to point your existing records (o

No easy answer here. Individual MTAs or a cluster of them typically live under one unique domain. In your scenario, you'd have to point your existing records (or just MX) to your self-hosted instance, and have your self-hosted instance relay/autoforward to Gmail under a different domain. This might entail simply setting your Gmail back to @gmail.com.