Attempts to push spam through apache

My apache logs are recently full of entries like these:

211.100.24.173 - - [19/Aug/2004:21:03:48 -0400] "CONNECT 208.17.33.40:25 
HTTP/1.0" 200 1844

Obviously this is an effort to pump spam through my server to 208.17.33.40. 
  There are many other target addresses as well.

If I telnet to port 80 and enter the HTTP command

	CONNECT 208.17.33.40:25 HTTP/1.0

the server replies with the 1844-byte home page of this site, as indicated 
by the "200 1844" part of the log entry.  As far as I can tell, this means 
that these exploit attempts only get a web page in reply and are not able to 
push the spam through to the intended target.

I don't have mod_proxy enabled or anything else that would enable proxying 
to work.  Are these just random spammer attempts to find an open proxy?  The 
fact that there are nearly 35,000 (!) such entries over the past few days 
suggests that the spammer, or the spammer's software, thinks this exploit is 
succeeding.  How can I be sure that it's not?

I've blocked the 211.100.24.0/24 subnet for now, but I'd like to be certain 
that others can't use the same exploit.  I tried a variety of Google 
searches but haven't found a useful page to read on this subject.

Some months ago someone used the recent mod_ssl vulnerability and managed to 
install an IRC proxy on this server.  However I fixed those problems at the 
time, and there's no evidence that any unauthorized programs, e.g., proxies, 
are now running.  (No, there are no rootkits installed, nor is the ps binary 
compromised, etc.  I'm well aware of such possibilities.)  Perhaps the 
machine was just added to a list of potentially vulnerable servers, and 
someone else is trying to take advantage of me, even though it's no longer 
possible?

FWIW, I'm running Apache 1.3.27 on RedHat 7.3, but I'd guess these types of 
exploits only work if there is an open http proxy available, no?


Peter