Email service providers
smtp;450 4.2.1 The user you are trying to contact is receiving mail at a rate that prevents additional messages from being delivered. Please resend your message at a later time. If the user is able to receive mail at that time, your message will be delivered. For more information, please visit https://support.google.com/mail/?p=ReceivingRate - gsmtp
smtp;450 4.7.3 Organization queue quota exceeded. [outlook.com]
smtp;450 User is receiving mail too quickly
smtp;450 4.2.1 The recipient has exceeded message rate limit. Try again later.
Spam filters services
smtp;450 cuda_nsu 4.7.1 <firstname.lastname@example.org>: Recipient address rejected: Policy Rejection- Abuse. Go away.
smtp;450 Requested action aborted  266-23, please visit http://www.symanteccloud.com/troubleshooting for more details about this error message.
smtp;450 4.2.0 <email@example.com>: Recipient address rejected: example.com greylisted for 60 seconds by ZEROSPAM