From: Walter Pinto on
Although untested, here is how I set mine up, can anyone confirm this
would work for the OPs question?

anvil_rate_time_unit = 60m
smtpd_recipient_limit = 100
smtpd_recipient_overshoot_limit = 20
smtpd_client_message_rate_limit = 100
smtpd_client_recipient_rate_limit = 100
smtpd_client_connection_count_limit = 50