Spam score is a metric used to assess the likelihood that an email or a web page contains spammy or unwanted content.
It uses algorithms that examine for characteristics like unreasonable hyperlinks, poor formatting, and certain keywords associated with spam. It also checks if your email address is authenticated.
The lower this measurement is, even negative, the better for your email deliverability
and for your email to be seen by your recipient. Anything above a 3 should not be sent out, and above a 5 the email will effectively stay unsent, and the greater the odds of being filtered out by anti-spam systems.