Once TXT record which contains DKIM has been added to DNS zone a special code is added to the headers of outgoing e-mails.
Receiving servers compare these headers with the information in DNS zone and if it matches then the e-mail is delivered.
450 – The requested command failed because the user’s mailbox was unavailable (such as being full). 451 – The command has been aborted due to a server error.
252 – The recipient cannot be VRFYed, but the server accepts the message and attempts delivery. This indicates that the server is ready to accept the message itself 421 – The service is not available and the connection will be closed.
You can see all of your SPF records from your domain’s control panel. Note that the merged SPF record contains, “a”, “mx”, and “~all”. Even if one of your SPF records has “-all” or “ all”, merging records will require it to be “~all”.
DKIM, or Domain Keys Identified Mail, is an email authentication method designed to detect email spoofing by providing a mechanism to allow receiving mail exchangers to check that incoming mail from a domain is authorized by that domain's administrators.
In the first case, recipient mail server looks up SPF record for your domain, and if it is not added / does not match actual outgoing server IP address, such a mail delivery will fail.
Such checking mechanism is implemented in order to make sure email comes from a legitimate sender and verified sender.
Domain Keys(DK) are not available on our shared servers as DK implementation was converted to DKIM and extended in a number of ways as of c Panel 11.32 and later releases.Please ensure that the DKIM value is exactly as written in our email setup page. The line of numbers, letters, and other characters that follows is called the Key.