Get list of the IP addresses from /var/log/maillog which are trying to send mail to unknown user:

# grep -A1 "User unknown" /var/log/maillog | sed -n 's/.*\[\([0-9]*\.[0-9]*\.[0-9]*\.[0-9]*\)].*/\1/p'

190.238.202.226
201.219.199.50
201.255.168.32
203.190.215.209
84.232.162.62
201.205.254.118
190.233.172.167
24.231.49.46
204.252.154.113
80.14.151.159
151.15.20.40
151.15.20.40
213.180.29.195
2.226.32.189
90.190.222.150
88.248.101.247
88.248.101.247

My spammers list getting bigger.