I was one of the early adopters of the free G Suite (formerly Google Apps) package from Google. So I set up a little custom domain and email for my family. It's limited to 50 users but that's way more than I need or use.
Just in the past few weeks, many of the emails I send out by myself (eg, not replies) get sent to my recipient's spam folders. Ironically, even a family member who is on the same custom domain name that I'm on! All the people where this has happened have happened to be on Gmail but those are just the people I'm aware of.
I have no problem receiving other people's emails.
I am not a marketer and I only use the email for personal purposes -- nothing automated has access to my account. I have never had this problem until just recently.
What can I do?
Update: as per the first response below, I had not ever set up SPF, DKIM, and DMARC. I have now done so. After a few days, the problem stopped.