One thing I decided to do after the switch to FastMail is to do a daily triage of my Spam folder:
1. Go into spam and scan everything looking for non-spam. Non-spam, as I define it, is: (a) a real email from someone to me, or (b) a mailing list I intentionally signed up for.
2. Mark all non-spam as such and delete the rest.
3. Unsub from 1(b) if needed, and reply to 1(a)
This process has caught important emails that were mis-categorized. Doing it daily makes it manageable.