Email messages are accumulating in the Messaging Gateway (SMG) delivery queue with the SMTP status
451 4.7.650 The mail server [ip address] has been temporarily rate limited due to IP reputation. For e-mail delivery information, see https://postmaster.live.com (S775) [hostname]
Release : 10.7.4
Email delivery from the Messaging Gateway server to outlook.com has been rate limited.
Messaging Gateway is operating as designed; it is the destination mail server which is deferring message delivery from the SMG system in order to slow down message delivery.
Queued messages will be delivered at a later time once the destination mail service removes the rate limit and accepts the email.