Most spammers are very aggressive in terms of connection and re-connection to mail servers. Of course they want to send as many SPAM as possible in the shortest time period.
However if you’re using postfix you can throttle that by being a bit more restrictive in your anvil settings in
smtpd_client_connection_count_limit = 2 smtpd_client_connection_rate_limit = 1 anvil_rate_time_unit = 45s
Found at policyd stats