Most inbox problems come from a few repeat offenders: weak authentication, shouty subject lines, messy content balance, and poor sender reputation. Samuel checks those because they are the things people can actually improve.
Mailbox providers want proof that the message really belongs to the domain in the From line.
Spam filters and people both react badly to subjects that sound like panic, hype, or manipulation.
Emails that are all image, all hype, or mostly links tend to look like campaigns filters were built to stop.
The best technical setup in the world will not rescue mail that recipients do not want.
Subject: Invoice #24936 for April work
Why it helps: specific, calm, and expected.
Subject: ACT NOW!!! LIMITED TIME OFFER
Why it hurts: all-caps, hype language, and repeated punctuation.
Write like a real person with a clear reason for emailing, not like you are trying to trick attention out of someone.
Samuel is most useful when you treat the score as a conversation starter, not a magical verdict. The point is to show what looks risky and why, so you can improve the next send.