[amaccuish]

I've never had that. My setup mostly did DKIM and all that for me. Never had mail arrive at the other end into spam. For incoming, I use rspamd. It's taken some training but it's a lot better now.

[txtsd]

My SPF, DKIM, and DMARC are on point. No idea why it goes to gmail's spam.

[acct1771]

Anticompetitive practices designed as strong security.

[krageon]

It's possible your emails are malformed or contain weird headers, that gets them flagged very heavily as well.