Does you email get marked as SPAM by gmail?

I do something similar, but when I send an email to a person for the first time, it always ends up in the spam folder by GMail.

Just relay it out via a trusted host, in addition to setting up DKIM, SPF, etc.; that solved all the problems for me. My email server is working excellent for many years now (OpenBSD, OpenSMTPD, dovecot) with Rainloop as web-based frontend.

I have DKIM, SPF, etc. but still gets caught by spam. What do you mean by "trusted host"?

An email provider that is not blacklisted and allows relay. I currently use my ISP for that.

Taken from my smtpd.conf:

  table secrets { mylogin = [email]@ziggo.nl:[pass] }
  accept tagged DKIM for any relay via tls+auth://[email protected]:587 auth <secrets>

Oh, I see, thanks!

It kind of defeats the purpose, since now the third-party will be able to read all my outgoing emails, and have control over them.

You're transiting over their network anyway -- the jig is already up even without relaying via their gateway.

Connecting directly to the remote (non-ISP) mailserver with TLS shouldn't reveal any message contents.

Relaying via the ISP's mail server though (even with TLS) seems like it would disclose the message contents.

Is DMARC included in your 'etc'? I think thats needed for gmail these days. If rdns, dkim, spf and dmarc are there, try checking your IPs reputation (senderscore) that could be it.

Thirded, only with qmail. I know mine is a tiny private server, but scaling up would only require more hardware and minimal increase in hours working on it. OTOH, I manage hosted mail solutions for three other domains, and those all require more time, because I'm supporting other people.

This used to work for me but lately, way too much spam. Looking to install rspamd but have not succeeded yet.