On Sun August 9 2020 15:52:37 David C. Rankin wrote:
> # iptables -nvL --line-numbers
> Chain INPUT (policy ACCEPT 3651K packets, 548M bytes)
> num   pkts bytes target     prot opt in     out     source
> destination
> 11    3074  182K DROP       all  --  *      *       37.0.0.0/8          
> 0.0.0.0/0

Dang.  I checked as much as I could of your SMTP and even checked
BGP looking glasses but of course I couldn't see your iptables!

You might want to try rate limiting, connection limiting, and
fail2ban instead of hard iptables blocks.

--Mike