What is a spam content check?
A spam content check runs a message's subject, text and links through a filter's content rules. Apache SpamAssassin is the widely used filter whose rules and scores are public, so this check runs those rules with their published scores. Content is one part of filtering: authentication and sender reputation are judged too.
Reading the result
Enter a subject, paste the body as HTML or text, or both. Each rule that matches is listed with its name, SpamAssassin’s own description, the text it matched and its score, then the total of those scores. The total is not a SpamAssassin score: a real scan adds header, blocklist and Bayes tests, and calls a message spam at 5 points by default.
Common problems it finds
An image with almost no text
SpamAssassin's HTML_IMAGE_ONLY rules score short HTML that contains an image, by up to 2.8 points. An email that is one big image also gives filters and screen readers nothing to read.
Capital letters
UPPERCASE_75_100 scores 1.48 when more than three quarters of the body's letters are capitals, and SUBJ_ALL_CAPS scores 0.5 for a subject written all in capitals.
Stock salutations and removal lines
DEAR_FRIEND (2.683), DEAR_SOMETHING (1.999) and EXCUSE_REMOVE (2.907) target the stock phrasing of bulk mail. Writing to a named person avoids them.
Links to IP addresses and shorteners
Links to a bare IP address match NUMERIC_HTTP_ADDR and NORMAL_HTTP_TO_IP, and a tinyurl.com link matches FSL_HAS_TINYURL, which scores 3.299.