One of my client is hosting their own exchange 2000 server and is not receiving email from two of their customers. This seems to only happen when the customers reply to the email. No bounced back or ndr are sent back to the customer. So the customers thinks the email is being sent to my client succesfully.
I have done a dns report on their domain name and can only find two issues. But I think the issues below only relate to emails sending emails not receiving them.
WARN Mail server host name in greeting WARNING: One or more of your mailservers is claiming to be a host other than what it really is (the SMTP greeting should be a 3-digit code, followed by a space or a dash, then the host name). If your mailserver sends out E-mail using this domain in its EHLO or HELO, your E-mail might get blocked by anti-spam software. This is also a technical violation of RFC821 4.3 (and RFC2821 4.3.1). Note that the hostname given in the SMTP greeting should have an A record pointing back to the same server. Note that this one test may use a cached DNS record. mail.lesterblades.com.au claims to be host 192.168.1.254 [but that host is at 192.168.1.254 (may be cached), not 126.96.36.199].
WARN SPF record Your domain does not have an SPF record. This means that spammers can easily send out E-mail that looks like it came from your domain, which can make your domain look bad (if the recipient thinks you really sent it), and can cost you money (when people complain to you, rather than the spammer). You may want to add an SPF record ASAP, as 01 Oct 2004 was the target date for domains to have SPF records in place (Hotmail, for example, started checking SPF records on 01 Oct 2004).