Monday, 29. November 2004
Greylisting the hard way
The short version - it doesn't work.
The long version. It does work - rather well as it happens with a c. 30% reduction in spam here as measured by items trapped here during the period it was on compared with equivalent periods before it was on. But... Two complaints about false positives later, it has been turned off.
The long version. It does work - rather well as it happens with a c. 30% reduction in spam here as measured by items trapped here during the period it was on compared with equivalent periods before it was on. But... Two complaints about false positives later, it has been turned off.
- Complaint 1 - a severe delay in email delivery - as it turns out, some MTA software is rather badly behaved and in at least one case (I suspect a certain older version of MS Exchange - most contemporary Exchange servers seem to be able to handle this), the sending host connects first to the "deaf" preferred MX and gets a 421 transient failure. Within seconds, it reconnects to the real MX and gets a 220 greeting (meaning "continue, I'm listening") but bails out immediately as if it has also received a 4xx response there (has the earlier 421 cached?). This goes on at intervals of a few minutes for up to 24 hours before the remote server suddenly connects to the real MX, receives a 220 greeting and actually understands what is being said, so goes on to deliver the email.
- Complaint 2 - a hard fail - one remote MTA (I suspect home grown, probably implemented in PERL or something like it) just never tries any MX other than the preferred one. It does this for 24 hours without success (or a permanent failure either) before giving up and returning the email to the sender with a bogus 554 message worded to the effect that "I tried one of the three MXes for this domain for 24 hours and never connected so I must have been rejected".
Another perfectly good spam killer foiled by lazy/incompetent MTA software. However, your milage may vary so if you can, the idea may well still be worth a try.
Category: Domino: Administration
Technorati: Domino: Administration
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