PTR - Due to Mis-configured DNS Settings


Unable to send email to an email address hosted by Typically a bounced message is received with one of the following errors: 

  1. Remote Server returned '554 5.0.0 < #5.0.0 smtp; 554 Sending address not accepted due to spam filter>'
  2. 501 Domain must resolve.
  3. Delivery to the following recipients has been delayed. 
  4. The following recipient's could not be reached: <email address> Could not deliver the message in the time limit specified. Please retry or contact your administrator.
  5. There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.  Connecting IP must have valid PTR record for (sender's mail server IP)> 

Cause requires all servers delivering email to to have properly configured DNS. A reverse DNS lookup is performed on the server delivering email and if there is no reverse PTR name the email is rejected. To guard against spoofing, then does a forward lookup on the PTR name to make sure it matches the IP. If it returns a different IP the email has its spam rating is increased, which may lead to message rejection.

Example with correct DNS: 

  1. Incoming mail connection from
  2. Reverse lookup of returns so the lookup continues.
  3. A forward lookup of returns an address of
  4. Since the original IP and forward lookup IP addresses match email is accepted.


  1. Forward this information onto your ISP or IT department to fix the DNS records. 
  2. Make an A record pointing to your outgoing mail server.
  3. Make sure the above A record has a reverse PTR that is identical. 
  4. The fix can be confirmed using MXToolBox's reverse DNS search tool.

To use MXToolBox to verify your DNS setup:

  1. Visit
  2. Enter the IP address of your mail server.
  3. Click on the Reverse Lookup button.
  4. When corrected a PTR record will be displayed and messages from your system will pass the PTR record check that is used on many email servers.