Mail rejected or flagged as Spam
When a problem occurs delivering your message to its destination on our servers, you will receive an error message from our system, either in your mail server's logs, or as an undeliverable / bounce-back.
The information below will give you more details on the meaning of the error message and the steps you can take to solve the problem.
Error message :
554 5.7.1 Service unavailable; Client host [xxx.xxx.xxx.xxx] blocked.
550 5.1.1 <email@example.com>: Recipient address rejected: User unknown in virtual alias table
450 4.2.0 <firstname.lastname@example.org>: Recipient address rejected: Greylisted, see http://wwbcity.com/help/wwbcity.com.html;
Reverse DNS and the requirements :
Reverse DNS is a way of associating an IP address with its hostname. The reverse DNS identifier is contained in the PTR portion of the IP Zone File. The IP Zone File contains all the different ways that your IP and domain name can be associated; each association serves a different need.
We require that all connecting Mail Transfer Agents have established reverse DNS, regardless of whether it matches the domain.
Reverse DNS must be in the form of a fully-qualified domain name. Reverse DNS containing in-addr.arpa are not acceptable, as these are merely placeholders for a valid PTR record. Reverse DNS consisting of IP addresses are also not acceptable, as they do not correctly establish the relationship between an IP address and its associated domain's.
Any host over the Internet is supposed to have a valid reverse DNS (PTR Resource Record) declared, as required by RFC 1033: Domain administrators operations guide,
Verifying that your DNS conforms. Ensure that your PTR and A records are visible by the rest of the world over the Internet, as sometimes they appear fine internally within your organization, but are not propagated over the Internet due to a delegation failure. Use one of the many free web-based tools available over the Internet to verify your reverse DNS records as they are seen by the rest of the world.