Several times a day, exim4 gets a connection that tries 100 different RCPT commands

I have bad actors attempting to figure out valid user names on my exim4 server. Many times a day, I'll get a connection that tries 100 RCPT commands with random-ish account names. They have the correct domain, and plausible user names, but so far, they haven't hit on a valid user name. This all happens in less than one second. It seems to be exactly 100 attempts every time. How can I limit how many bad RCPT commands can be issued per session? These attempts originate at different IP addresses every time and happen so fast that fail2ban can't help. Thanks