That ip range is not inside your office:
You need to block that through a firewall or router's DOS protection tools. It's a good reason to have your server on a DMZ behind a firewall--makes filtering this kind of stuff a lot easier.
$ whois 18.104.22.168
OrgName: State of Georgia/Board of Regents
Address: 2500 Daniells Bridge Rd
Address: Building 300
NetRange: 22.214.171.124 - 126.96.36.199
NetType: Direct Allocation
RTechName: University System of Georgia
OrgTechName: University System of Georgia
# ARIN WHOIS database, last updated 2008-03-05 19:10
# Enter ? for additional hints on searching ARIN's WHOIS database.
You can do it with the linux firewall on your server, but be careful if you try that route--I've seen more than one user turn on his mailserver firewall and forget to open crucial ports (like, for example, allowing 7071 from the internal network) which makes it REAL hard to administer. Most admins would, I think, consider it best practice to have your firewall on a separate box. . .