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block dos SMTP using iptables

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http://www.linuxquestions.org – I've got a server that is getting hit hard on port 25 from random IPs with what appears to be complete garbage. I'm wondering is there a way to use iptables to drop these packets while allowing legitimate traffic through as the sendmail server is overloading the server. an example of this data is: 13:28:11.230871 IP (tos 0x0, ttl 64, id 20086, offset 0, flags [DF], proto: TCP (6), length: 834) [servers IP] .smtp > 89.136.193.161.comcam: P, cksum 0xb260 (correct), 2523791976:2523792770(794) ack 1231612831 win 7168 0x0000: 4500 0342 4e76 4000 4006 5d7a d875 9826 E..BNv@.@.]z.u.& (HowTos)