What are the limits for email recipients in Exchange Online?

The new external recipient rate limit introduced by Microsoft is meant to put an end to mass mailing using cloud mailboxes in Exchange Online. Read on to learn when it will be enforced, how it exactly works and what other recipient limits apply to Exchange Online.

External recipient rate limit in Exchange Online

External recipient rate (ERR) limit in Exchange Online

In April 2024, the Exchange Team announced the new external recipient rate limit in Exchange Online:

2,000 recipients / 24 hours / mailbox

The limit is an extension of the recipient rate limit that’s already in place (I’ll get into that later). After any mailbox sends emails to 2,000 recipients within a 24-hour window, it will be blocked from sending external emails and will get non-delivery reports. This limit is not yet live, but it will be soon enough:

  • Starting 2025, the limit will be introduced for all cloud-hosted Exchange Online mailboxes of newly created Microsoft 365 tenants.
  • In Q3-Q4 2025, the limit will be applied gradually to existing tenants.

Those limits are difficult to reach for standard users. However, there are legitimate scenarios in which the possibility of reaching the limit is real. Popular examples include service accounts, if they are set up to send a lot of automatic emails, or if you misconfigure the notification settings in any tool capable of sending emails. Also, when using tools like ticketing systems, you will need to take extra care to use SMTP relay and avoid sending emails by using cloud-hosted mailboxes.

And if you use Exchange Online mailboxes for mass mailing (which you shouldn’t), you’ll be able to send 5 times less external emails than you can send now.

How are recipients counted?

Limits are based on recipients added in the To, CC and BCC fields of an email. Now, the interesting part is how Exchange Online counts distribution lists (DLs). A DL in a Global Adress List (so, an organization-managed DL) is counted as one recipient. A DL stored in the contact folder (so, a DL created by an individual user) counts all group members individually.

So, theoretically, you could go around the recipient-rate limits (both external and general) by using DLs.

That being said, to get around the external limit, it would require admins to create and manage large volumes of external contacts in your Exchange Online environment. Also, it doesn’t change the fact that Exchange Online mailboxes simply aren’t meant for mass mailing and the ERR limit is meant to put an end to bulk emailing via cloud-based mailboxes. If the limit doesn’t stop Microsoft 365 tenants from sending mass emails, next steps are sure to follow.

What about mail flow connectors?

You can have mail flow connectors (for example, for applying cloud email signatures) that route email to an Azure service and then back to your tenant. In some scenarios, this could mean that internal emails are treated as external, since the email leaves your tenant. In the face of the new external recipient rate Limit, it could potentially bring your overall recipient rate limit to 2k emails per 24-hour period.

Fortunately, Microsoft accounted for those scenarios, and internal emails processed by connectors will be treated as internal.

However, if you’re using a connector to a 3rd party gateway for mass mailing, the same ERR limit will apply. The connector can help you work around the message rate limits, though.

More recipient-related limits in Exchange Online

But wait, there’s more! Here are some other Exchange Online limits that apply to recipients.

Recipient rate limit

10,000 recipients / 24 hours / mailbox

The limit applies to any mailbox over any 24-hour period. It doesn’t reset at any specific point of the day but continuously looks back at the past 24 hours. If you reach 10,000 recipients (both internal and external) or 2,000 external recipients within any 24-hour span, you’ll get NDRs and will need to resend the undelivered messages after some time has passed.

Recipient limit

Up to 1,000 recipients / email

This limit is configurable per tenant. There’s no distinction between internal and external recipients here.

Message rate limit

30 emails / minute

This is how many emails can be sent from a single mailbox. Now if you manage to break this limit, the excess emails will be throttled and carried to the following minutes. Better than receiving NDRs and having to resend emails, right?

This limit doesn’t apply if you use Direct send or SMTP relay, which are explained in more detail in this Microsoft article. However, those methods do require custom SPF records to bypass antispam mechanisms.

Maximum number of recipients added by mail flow rules

100 recipients / email

This sums up all mail-flow rules that apply to a single message.

Maximum number of forwardees added by mail flow rules

10 recipients / email

This limits the number of forward recipients added by mail flow rules to a single email.

Receiving limit

3,600 emails / mailbox / hour

No mailbox in your tenant can receive more emails than that, from any source. If it’s exceeded at any point, further delivery will be blocked.

Receiving limit from a single sender

1,188 emails / hour

This value is the 33% of the receiving limit.

So, how does that affect me?

For most companies, those limits would be hard to reach, even if end-users tried really hard. I mean, receiving more than 3,600 emails per hour? Typically, you’d have to use a free email account and publish your email address on a website with no bot protection to reach that level of traffic.

It’s like with those seemingly absurd warning signs, like “do not iron while wearing” on shirts or “do not use to dry pets” on microwaves. There’s a story behind each one of them. Those limits are there for a reason. Some prevent email storms, while others protect against mail-flow rule based email archiving (which is not permitted anyway).

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