The best way to improve your SpamAssassin Score is to follow clean emailing practices:
Clean your mail list to avoid bounce. Don't use bought mail lists.
Set up authentication protocols (SPF, DKIM and DMARC).
Send relevant content to avoid being reported as spam.
Use List-Unsubscribe Header to make things simple for your subscribers.
Take care of URLs in emails (avoid broken links and shortened URLs).
Avoid using shared-IP to fully control your sender reputation.
If you need more information about your SpamAssassin Score and how to improve it, don't hesitate to reach us on our live chat.