To keep the reputation of your Mail Transfer Agents healthy it is a good advise to set a Recipient Rate Limit on your Exchange Servers. With this measure you are able to prevent spamming and the transmission of bulk mails ( generally new-letters ).
This can be achieved through a Throttling Policy where you set the Recipient Rate Limit to a value which fits your customers mail sending practices, but prevents a spam critical amount of outbound mails.
In this example we will set the value to 2000 mails / 24 hours :
Apply the Recipient Rate Limit Exchange Server wide on all mailboxes : ( Exchange 2013 / 2016 )
New-ThrottlingPolicy -Name Limit2000 -RecipientRateLimit 2000 -ThrottlingPolicyScope Organization
Apply the Recipient Rate Limit for a certain organization : ( Hosted Exchange 2010 Multi-Tenant)
Get-ThrottlingPolicy -Organization “TenantName” | New-ThrottlingPolicy -Name Limit2000 -RecipientRateLimit 2000
Apply it on an Exchange 2010 OnPremise :
New-ThrottlingPolicy -Name Limit2000 -RecipientRateLimit 2000
Remember not to alter the Default Throttling Policy since this is the template for all further Throttling Policies you create.
To apply a Throttling Policy on mailbox level please see the following post :