We run your emails through SpamAssassin, a very popular spam ranking software, to check for known spam email red flags.
It's mostly informational, to help you stay clear of things that are going to trigger spam filters based on SpamAssassin, although if you have an exceptionally high spam score, we will refuse to send your email.
It is not a reflection of how many people reported your email as spam, it's only about how your email is being ranked by the SpamAssassin software.