I posted last year of valid emails being put to Spam folder (mail forum I subscribe to). I followed advice at time and it has not resolved the issue. I only found out by using Evolution on my Desktop running KDE neonuser.
There is a guide that could be helpful with this problem.The team did a study to allow a feature where users could on their own accounts set up blacklist and whitelists specific domains. This feature will require to be planned for implementation. Will update if there is any progress on this.