Are you running big instance in terms of users and traffic?
Anyway, logs told you what to do. Unfortunately mailcow strip usual first few lines of master.conf
/etc/postfix/master.cf:
# ====================================================================
# service type private unpriv chroot wakeup maxproc command + args
# (yes) (yes) (yes) (never) (100)
# ====================================================================
as you see, there is “maxproc” part that is by default “(100)”. Mailcow smtpd service use “-” in maxproc so use default that is 100. As your logs told you, you are on upper side of this limit, so you can add more processes to your smptd service. But it’s a part of huge machine so dial one cog could change behavior of a whole machine.
So some reading that should help
https://www.postfix.org/master.5.html
https://www.postfix.org/TUNING_README.html
https://www.postfix.org/STRESS_README.html
In this case I would try to pay mailcow support and solved it using professional who know Mailcow and Postfix from inside out.
https://www.servercow.de/mailcow?lang=en#support
