What is a subject line score?
A subject line score is an estimate of how a subject reads to spam filters and to people scanning an inbox. No filter publishes its rules, so the score is built from patterns that correlate with filtering: length, capitals, punctuation, commonly filtered words, emoji, fake reply prefixes and merge tags left unfilled.
Reading the result
The score runs from 0 to 100 and is a blunt measure, labelled as one. The list under it matters more: each pattern found is named with its reason, so you can disagree with any single point. Length is shown too: inboxes cut a subject by its width on screen, not a fixed number of characters, so a phone hides more than a desktop, and the tool flags anything over 78.
Common subject line problems
A fake reply prefix
Opening a first message with Re: or Fwd: pretends to continue a conversation that never happened. Readers notice, and some filters check for a reply prefix with no matching thread headers.
A merge tag left in
A {{first_name}} or %%company%% in a sent subject shows the reader your template. It usually means that field was empty for the contact.
Capitals and exclamation marks
A subject that is mostly capitals, or has two or more exclamation marks, reads as shouting. Both are long-standing spam patterns.
Commonly filtered words
Words like free, guarantee or act now appear in plenty of legitimate mail, so one alone is only a note. Three or more together raise it to a warning.