traditionally, our (pre-PMG) exim4-mailserver has used spamassassin's `$spam_bar` to indicate the spam-level with `+` signs, e.g. X-Spam-Score: +++++++
now PMG uses `__SPAM_STARS__` instead, which is similar in concept, but uses a different characters (`*` instead of `+`), e.g. X-Spam-Score...