Looking back I find that I first installed the MT-Blacklist plugin about a year ago. Since then the amount of comment and trackback spam has done nothing but increase, seemingly with no end! I really noticed this after a period of not using MT-Blacklist; every day there were more and more spams to be deleted, and ips to be banned. I started using MT-Blacklist again about 3 weeks ago; it's made quite a difference, as these stats show:

MT-Blacklist Stats
Comment spams blocked: 3103
Comment spams moderated: 455
Duplicates blocked: 0
Blacklist - Strings: 2476
Blacklist - URLPatterns: 33
Blacklist - Regexes: 24
Blacklist - FlexProtect: 0
Blacklist - Pending: N/A

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