Author: Craig Jackson Date: To: Willie Viljoen CC: exim-users, I. Forbes Subject: Re: [Exim] OT: Problem sending mail to verizon.net - Follow up
This leeds me to believe that these servers perform a practice known as
>sender verification. Though not uncommon, it is combersome and creates long
>wait times and alot of DNS traffic overhead. Basically, once you send the
>MAIL FROM: command, the server first performs a DNS lookup on your domain,
>once it has found the domain to exist and have a valid MX record, it
>connects to the MX listed and attempts to send a mail back to the sender,
>disconnecting just after RCPT TO: was accepted.
>
>This helps to prevent spam, but causes several problems. I have a hunch
>there is probably some issue with DNS or your primary MX, or perhaps a
>connectivity issue between you and verizon which does not allow the verizon
>server to connect back to yours. This could occur if a major overseas line
>is down and asymetric routing is being used to compensate.
>
>I don't know what else it can be. Check your DNS and mail servers, and phone
>your ISP and make sure their network is functioning properly.
>
>Will
>
>
> Recently there was a proposal of sorts suggesting a practice such as you
describe and it was called "gray listing". The purpose was to cut out
spam. One problem is the overhead per email and additional problems
therewith. Other problem is the DDOS spam where the FROM addresses are
mostly valid making it a waste of time, bandwidth and general PIA.
