A spam score is a number estimating how likely an email is to be filtered as spam. The best-known comes from SpamAssassin, an open-source filter that adds points for each suspicious feature; mail with a total above a threshold (often 5) is marked as spam.
What adds points
Missing or failing authentication, links to domains with a bad reputation, all-caps or exclamation-heavy text, typical spam phrases, an image with little text, and HTML that doesn't match the plain-text version.
How much to trust it
Gmail, Outlook and Yahoo don't use a public spam score; their filters rely mostly on reputation and recipient behaviour. A good score doesn't promise the inbox, but a bad one points to fixable problems in the setup or the email.
Better checks for cold email
An inbox placement test shows where an email actually lands at each provider. For the email itself, the free spam word checker catches wording and formatting filters dislike.