[NCLUG] Apparently, I'm a Spammer - Now What?

[Rich Young]

I just noticed our web server running unusually slowly, and a little
digging turned up a huge load on sendmail.  It didn't take long to find
record in the maillog of a lot of spammy activity, which apparently
began Monday morning.

Sendmail on this box isn't mission-critical, so I shut it down
immediately.  But that's not a viable long-term solution, so I'm faced
with the prospect of figuring out what the heck happened and why.  

Does anyone have any concise, helpful advice to someone who might be in
over his head?  

CentOS 4.4, sendmail 8.13.1-3.

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I'm Rich Young, and I approved this message.