stop flooding of smtp / block spammers

/ ip firewall filter
add chain=forward action=add-src-to-address-list dst-port=25 protocol=tcp \
src-address-list=spammer address-list=WasASpammerOnce \
address-list-timeout=0s comment="Log Spammer to address list for future \
investigation" disabled=no
add chain=forward action=tarpit dst-port=25 protocol=tcp \
src-address-list=spammer comment="BLOCK SPAMMERS OR INFECTED USERS" \
disabled=no
add chain=forward action=add-src-to-address-list dst-port=25 protocol=tcp \
connection-limit=30,32 limit=50,5 src-address-list=!WhiteListed \
address-list=spammer address-list-timeout=30m comment="Detect and add-list \
SMTP virus or spammers" disabled=no

soure:http://forum.mikrotik.com/viewtopic.php?f=7&t=21836