Requirements For Message Delivery To

This article is intended for email administrators and other IT professionals. uses the following measures to protect members from unsolicited bulk e-mail (UBE or spam). Messages and mail servers that fail these controls may be refused by's e-mail system.

  • utilizes several real-time blacklists to block connections. These blacklists include open relays, open proxies, or any other systems that has been determined to be available for unauthorized use. Please click here for the current blacklists that uses. 
  • Connections that do not follow the DNS standards of RFC 1912, section 2.1 will be rejected. Please refer to this KB for more details: PTR - Due to Mis-configured DNS Settings
  • Messages without a valid or properly formatted "mail from" address will be rejected. 
  • uses content filtering to identify spam, and may reject connections from IP addresses or domains that fail many content filter checks. 
  • may refuse connections from servers whose recipient lists generate a high volume of bounce/undeliverable failures. 
  • may limit connections from servers that send a high volume of messages in a short amount of time. 
  • checks servers for SPF requirements. The message is rejected if the IP address of the server delivering email does not match the SPF requirements for the domain. Please refer to this article for more details: SPF Rules. 

When rejects a connection or message, a status code or error message is returned to the sender that indicates the cause of the rejection. This article explains the common status codes and error messages: Non-Deliverable Error Messages.