Once they find an IP that answers to port 25, they'll try everything they can to relay through it or guess valid email addresses that they can deliver spam to. I don't think you want to quit logging it just in case something does come through that you later need to diagnose.
fail2ban and iptables --recent would cut down on the log entries by firewalling the IPs that are attempting to connect multiple times. Each has a 'window' of opportunity for a spammer to try before getting blocked. fail2ban would be better in this case as it would block based on failure messages. iptables --recent is much less discriminate and might block legitimate connections that passed a threshhold.
You could alter your syslog configuration to log only .info to your maillog, and .warn/.err to a separate file.