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Obviously, every site is different. Here are some details on one large scale deployment that you may find useful as a benchmark.
\[http://www.mweb.co.za MWEB\] is the largest ISP in South Africa, with approximately 350,000 mailboxes, all of which get filtered through [SpamAssassin].unmigrated-wiki-markup Wiki Markup
Their setup entails 8 servers running Postfix, 12 servers running \[http://www.ijs.si/software/amavisd/ amavisd-new\], and an additional 5 servers running Trend Micro and Vexira anti-virus. Their 25 servers are Dell Power Edge 2650's with dual 3.0 GHz Xeons, 4 gigs of ram, and 5 SCSI drives in a RAID 10 configuration (1 hotspare). They're running a custom built Fedora Core 2, 2.6.7 kernel .
Here are their statistics for April:
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