
All spam emails, you receive, are from both known and unknown sources. Let me share a good example of emails from known sources. Few months I got my car serviced at a service center and upon their request, I filled the feedback form including my personal email ID. After few days I started receiving their promotional mails.
Steps to Stop Spam in Gmail:
1. User Specific blocking
2. Create filter with search
3. Report Google
4. Alias Tweaks