Spam mail filter problem
Many mail server admin use DNSblacklist on their server to reject e-mail from blacklist ip, without knowing the effect of false positive detection.
it cause problem with inbound mail that send from their real customer.
Imagine, top executive director missing one important e-mail from customer and He need that e-mail urgently
and he ask you for it.
It is not the situation you want to get involve at all.
practically, this is not the best solution to apply to your mail system, especially inbound mail filtering system.
it is better to accept every e-mail and sort it to separate spam folder.
this way end-user will never missing any false positive e-mail from their real customer.
Also opposite mail server admin will not have a hard time remove their ip from the DNSblacklist,
just to make single e-mail pass through blacklist filtering system.
it is time consuming and very slow response.
It should be the work on recipient mail server, not sender mail server.
This is why, sometime normal user send mail but recipient never receive it.
and then they start to blame each other mail server.