Microsoft's new sending limit for "onmicrosoft.com" subdomains

When an organization sets up a new tenant in Microsoft 365, an onmicrosoft.com subdomain is automatically provisioned to let customers start using the services right away, even before setting up a custom domain.

For example, if your organization is named Contoso, Microsoft will likely provide contoso.onmicrosoft.com as a default domain, which you’ll be able to use as a sender.

Microsoft says that these domains were abused and frequently used for sending spam messages, so it will now add new limits to email sent from these domains.

Specifically:

We will be introducing throttling to limit messages sent from onmicrosoft.com domains to 100 external recipients per organization per 24 hour rolling window. […]

External recipients are counted after the expansion of any of the original recipients.

When a sender hits the throttling limit, they will receive NDRs with the code 550 5.7.236 for any attempts to send to external recipients while the tenant is throttled.

While this should have always been the case, Microsoft recommends using the default onmicrosoft.com domain for testing purposes and not for regular email sending, which should instead use custom domains.

Refer to Microsoft’s blog post for more details about this change and on how to configure your tenant to never use the onmicrosoft.com default domain.