It's probably senders that haven't configured DKIM etc. so there is no way to verify the email is from an address marked as "safe" and not a scammer.
My beef is that marking a sender as "spam" doesn't work anymore. There's a hidden limit on the number of email addresses you can mark as "spam" with joogle mail, and there's no way to edit this list. The only alternative is to create a filter for really obnoxious senders.
gmail usually warns about that.
I was having a similar problem, mail was just ending up in spam. This was mail coming from my own service, I spent a lot of time making sure I had all the right records, and it had been coming from my service for years.
One day, gmail just started dumping stuff in spam. "You reported this message as spam from the web console" or something which was a complete lie because I use Thunderbird and don't log in to the web client unless there was an issue. Found mail all over the place, including stuff from 2014 that it had decided to file in archives.
I ended up just buying an extra address on my email service for the service mails my machines send me, and no problems since.
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