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.
But PTRresolving is in SPF comment in DNS records for that site. I have been emailing this company for a number of years and most recently as this past February with no problems.
Now all of a sudden I get the following message: Mail Enable: Message could not be delivered to some recipients.
There could be a lot of reasons, including that someone entered it wrong, or that someone intentionally entered a fake email address into your system (such as when you require a user to divulge an email address in order to receive a download, etc.).