Delayed Message: Valid email address
The email message has not yet been delivered even though the recipient's email address is a valid email address.
Contact the recipient another way (e.g., phone or text message) to confirm their email address.
If the email address you're using is correct and your recipient can receive emails to their inbox, then you could contact the email provider for help.
4.1.5: Official Definition
A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay of attempts to send the message. If this code accompanies a delivery failure report, sending in the future may be successful.
This mailbox address as specified was valid. This status code should be used for positive delivery reports.