Basically: I'm finding it impossible to reliably convince Google that email from my employees, to my employees, is not spam.

I run my own mail server, but most of my employees use Gmail. So I have forwarding set up: employee@dnalounge.com simply forwards to employee@gmail.com. When sending mail using their Gmail account, they set their From line to employee@dnalounge.com. (Google lets you do this if you jump through some hoops to verify that you can actually receive mail at that account.)

So when one of them mails another, the mail goes from their phone up to Google's SMTP server, with "From: A@dnalounge.com, To: B@dnalounge.com". That hits the dnalounge.com MX (my machine) and is forwarded back to "B@gmail.com" -- where it then ends up in B's spam folder, because Google (maybe?) thinks it's a "forgery".

I have proper DMARC and DKIM records ("dig TXT mail._domainkey.dnalounge.com; dig TXT _domainkey.dnalounge.com"). TLS is configured properly. I have SPF set to "v=spf1 a mx ptr ~all" (since I do, in fact, want my employees to be able to originate their mail on their ISP's servers.)

What do I have to do to make Google stop fucking me?

Allowing Google to host the dnalounge.com domain is not an acceptable answer. Not having "@dnalounge.com" in the From and To fields is not an acceptable answer.

Forcing all of my employees to use my own IMAP server instead of Gmail would be a moderately terrible answer.

Previously, previously, previously, previously.