I have one domain name with 3 email addresses forwarding a couple of emails per day (No more than 10 emails) and over the last weeks I noticed that Gmail marks them all as spam.
Is there any way to fix this issue? I had the same domain forwarding emails with another free provider for over a year and my emails were delivering fine.
It probably has to do with the fact you had a different service to forward the emails. Google isn’t totally clear about their spam filter for obvious reasons. However, because emails came from a different ip before, they could be marked as suspicious. To solve this issue, you can read this article from Google themselves about preventing spam marking:
The simplest possible solution is to mark the emails as not spam.