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My host is looking at implementing greylisting and having read the details of this site I have a concern that maybe someone could help with.

Imagine a new customer sends you an email. Your greylisting bounces it back. No problem so far, as the sender's server will just resend it. But, the sender's server is not configured correctly, doesn't resend and eventually the new customer gets a bounced back message.

Luckily your new customer also has your phone number, so they ring you and say I tried to email you but it got bounced back. Ah yes say you, that's because we have greylisting and your mail server is configured incorrectly. OK says your new customer, I'll check with our server admin.

5 minutes later the customer calls back. I've checked with our server admin and they say there's no problem with our servers, it's a problem your end.

Do you then argue with them explaining that there server admins are rubbish and should be sacked immediately???

OK, I've been a bit tongue in cheek, but the point I'm trying to make is you have two people speaking to each other, both of which have limited/non-existent technical knowledge. All they want to do is email each other. I don't think you can't expect end users to argue with new customers over a mail server configuration problem.

What would you suggest to be the course of action in this case?

Thanks

I have just joined this forum because I am experiencing in fact the hypothetical problem described by richyrich. I recently changed ISP's and I am finding that my emails are not arriving. My new ISP tells me the problem is that the sender's server is not configured correctly. They have gone ahead and whitelisted a couple of servers as a result of my problem, including my bank - a major bank, not some small organization - and google.com. One would think these organizations would have correctly configured email servers, but apparently they do not...?

I am concerned that there may be many other emails legitimately sent to me, that are being rejected inappropriately. I have no way of knowing that it is happening, and I can't ask my ISP to correct a problem I'm unaware of. How do other people and other organizations handle this sort of situation?

jlepowsky Wrote:I have just joined this forum because I am experiencing in fact the hypothetical problem described by richyrich. I recently changed ISP's and I am finding that my emails are not arriving. My new ISP tells me the problem is that the sender's server is not configured correctly. They have gone ahead and whitelisted a couple of servers as a result of my problem, including my bank - a major bank, not some small organization - and google.com. One would think these organizations would have correctly configured email servers, but apparently they do not...?

Most likely what you are seeing is that these sites have several outgoing smtp servers, organized in such a way that retries are not guaranteed to come from the same IP address as the initial delivery attempt. Perversely, this is apparently RFC2821 compliant, since the RFC only says you MUST retry, but does not specify that the retries have to come from the exact same IP address as the initial delivery attempt.

The less palatable option is to whitelist those sites, either by individual address or the whole subnet as near as can determined. This may be your only choice if the other end does not see the problem.

The other option is to convince the relevant site's admin that they need to set up their SMTP pool to retry from the same IP address. It has struck me that placing their servers behind NAT so they all appear to be the same official address would probably cure their problem and help conserve their IP addresses.

Hope this helps. I have a fairly formalized rant about this in the BSDCan malware paper (http://home.nuug.no/~peter/malware-talk/...twork.pdf) and a slightly less formal version in the PF tutorial (http://home.nuug.no/~peter/pf/), perhaps some of it can be useful in order to persuade these sites to at least take a look at their setups.