Forward Technologies does not accept email from mail servers that do not have a reverse look PTR (pointer) record for the mail server to prevent spammers and zombied machines from sending unsolicited email. 99% of the mail servers that do connect to our mail server has a PTR record in their DNS reverse lookup zone. if you receive a error 501 domain must resolve error then you need to contact your hosting and/or email provider and ask them to add a pointer (PTR) record for your mail server in their reverse DNS. As soon as they add the PTR record to their reverse DNS zone then you will again be able to send email.
It is the responsibility of your DNS administrator to add reverse DNS records for the IP address that you use.
You can verify that you do not have a reverse PTR record by using the reverse DNS lookup tool located on the link below, simply enter your mail servers IP address and it will check to see if there is a reverse DNS entry for your mail server. http://www.dnsstuff.com/
What is Reverse DNS? Read more here: http://www.dnsstuff.com/info/revdns.htm
Note that all Internet accessible hosts are expected to have a reverse DNS entry (per RFC 1912 2.1), and many mailservers (such as AOL) will likely block E-mail from mailservers with no reverse DNS entry.
RFC 1912 (RFC1912) see section 2.1 here
“Every Internet-reachable host should have a name. The consequences of this are becoming more and more obvious. Many services available on the Internet will not talk to you if you aren’t correctly registered in the DNS.
Make sure your PTR and A records match. For every IP address, there should be a matching PTR record in the in-addr.arpa domain (reverse dns lookup zone). If a host is multi-homed, (more than one IP address) make sure that all IP addresses have a corresponding PTR record (not just the first one). Failure to have matching PTR and A records can cause loss of Internet services similar to not being registered in the DNS at all. Also, PTR records must point back to a valid A record, not a alias defined by a CNAME. It is highly recommended that you use some software which automates this checking, or generate your DNS data from a database which automatically creates consistent data.”
We would also like to encourage mail hosts to employ SPF (sender permitted framework) to help reduce fraudulent use of domains for sending spam. More on SPF is located here: http://spf.pobox.com/index.html
Magical Productions creates SPF records for each domain that we host and also utilize SPF on our mail servers. The combination of not accepting email from mail servers without reverse PTR records and also use of SPF has reduced our incoming spam load to less than 1/2 of a percent whereas prior to implementing these two features 70% of incoming mail was spam or unsolicited email from fly by night spammers and scammers..