To throttle number of messages and rate of email sending for a user
RecipientRateLimit and MessageRateLimit parameters in a Throttling Policy.
MessageRateLimit controls the number of messages per minute that can be submitted.
When messages are submitted using Outlook or OWA, the messages will stay in the Outbox longer when the user submits messages at a rate greater than the MessageRateLimit parameter. The messages are deferred, and will eventually be sent.
RecipientRateLimit limits the number of recipients that a user can send toin a 24-hour period. The user will receive an NDR for messages in excess of the limit.
To create a new policy where the users can send to 30 recipients a day and no more than 1 message per minute, you would use this command:
“New-ThrottlingPolicy -Name LimitMessagesSent -RecipientRateLimit 30 -MessageRateLimit 1”
To assign it to a user, use this command:
Set-Mailbox -Identity user_alias -ThrottlingPolicy LimitMessagesSent