Receiving "421 Message from (x.x.x.x) temporarily deferred - 4.16.50" when sending email to Yahoo!
This article provides reasons why you could receive a delivery failure message containing the error "421 Message from (x.x.x.x) temporarily deferred - 4.16.50" when sending email messages to Yahoo! from an third-party service.
If you’re seeing the error "421 Message from x.x.x.x temporarily deferred - 4.16.50", in your SMTP logs (where x.x.x.x is your IP address), this indicates that emails from your mail server have been generating substantial complaints from Yahoo! Mail users.
This is a temporary error and your mail server may automatically re-try sending emails at a later time. If you are the administrator for the message content or mailing policies, we ask that you review outgoing messages for potentially objectionable content or practices.
Please know that Yahoo! is unable to disclose specific information regarding our filtering practices; however, we do publish recommended guidelines at this page for review.
If you are not the administrator of the mail server in question, please contact the administrator directly with the error message you are receiving.
We thank you for your understanding in this matter.