We know his SMTP server is working, because the recipients mail server is receiving the email and checking if the the From address is valid, to avoid SPAM. So the mail server attempts to check if his SMTP server knows of a user with your email address, if this fails, the email is dropped - SPAM is suspected. Here, the recipients mail server is looking up his domain, asking for the IP Address of his mail server (MX record), however, the DNS server for his domain sends back nothing or an illegal reply. I guess he does not have an MX record for this domain, or the IP address it contains is wrong. Please check the MX record!
This is a message from the recipients mail server:
"Server replied: 511 sorry, can't find a valid MX for sender domain (#5.1.1 - chkuser)".
Last edited by thecarpy; 11-15-2007 at 03:09 AM.