Mailisk uses SpamAssassin in the background for spam scoring. By default mailisk will accept every email, regardless of their score.
Every email received is sent through SpamAssassin and the score is attached to the email response, see Search Inbox on how to receive emails via API. The score is stored in the field
The score ranges from negative to positive decimal numbers. A higher number means the email is more likely spam, while a lower number means it is non-spam. You should always aim to have a low score.
Note that while a high score means the email has a higher chance that it will be marked as spam, a low score does not guarantee it will land in an inbox (this is dependent on the recepient: Google, Yahoo...).