Richard,
It seems Jan is referring to the case when a single user account is under a brute-force attack from a botnet. Every single IP in this case attempts not enough times to get noticed and blocked by BFM, and there might be hundreds and/or even thousands of IPs. The lowest values in CSF if 5 failed tries (after which CSF might block an IP if it's not whitelisted or skipped), so even CSF might miss IPs with less than 5 tries.
And if this is the case I'm not even sure how much long and how easy for reading would such an email be. That's rather doubtful.