Exchange Online Limits

All users are provided with a national N365 E3 Restricted license. Exchange Online has a standard set of limits – around sending, receiving and folders.

This article highlights some of the key Exchange Online limits that Local Administrators (LAs) should be aware of ahead of migration. A full comprehensive list of limits published by Microsoft can be found here.

For the vast majority of users, the upper Exchange Online limits should be sufficient. If you have any concerns or believe you may exceed the parameters, then please contact


When reviewing the Microsoft Guidance – please ignore limits relating to Address Book, Mailbox Storage, Journal and Transport Rules. These settings are custom to the NHSmail tenant

Sending and Receiving Limits

Receiving and sending limits are applied In Exchange Online to combat spam and mass-mailing viruses.

Receiving limits

Receiving limits apply to the number of messages that a user, group, or public folder can receive per hour. This applies for both messages received from the internet and from on-premises servers. When the receiving limit has been exceeded, any emails sent to that mailbox will receive a non-delivery report stating that the mailbox has exceeded the maximum delivery threshold. After one hour, the limit will refresh and the mailbox will once again be able to receive messages.

Feature National N365 E3
Messages received 3,600 messages per hour

Sending limits

Sending limits apply to the number of recipients, number of messages, and number of recipients per message that a user can send from their Exchange Online account.

  • Recipient rate limit: To discourage the delivery of unsolicited bulk messages, Exchange Online has recipient limits that prevent users and applications from sending large volumes of email. These limits are applied per-user to all outbound and internal messages.
  • Recipient limit: This is the maximum number of recipients allowed in the To:, Cc:, and Bcc: fields for a single email message.
  • Message rate limit: Message rate limits determine how many messages a user can send from their Exchange Online account within a specified period of time. This limit helps prevent over consumption of system resources by a single sender. If a user submits messages at a rate that exceeds the limit via SMTP client submission, the messages will be rejected and the client will need to retry.
Feature National N365 E3
Recipient rate limit 10,000 recipients per day
Recipient limit Customisable up to 1000 recipients
Message rate limit 30 messages per minute

Distribution Group Limits

These limits apply to distribution groups in your organisations shared address book.

  • Maximum number of distribution group members: The total recipient count is determined after distribution group expansion.
  • Limit sending messages to large distribution groups: Distribution groups that contain the number of members specified by this limit must have delivery management or message approval options configured. Delivery management specifies a list of senders who are allowed to send messages to the distribution group. Message approval specifies one or more moderators who must approve all messages sent to the distribution group.
  • Maximum message size for large distribution groups: If a message is sent to 5,000 or more recipients, the message size can’t exceed this limit. If the message size exceeds the limit, the message isn’t delivered, and the sender receives a non-delivery report (NDR).
Feature National N365 E3
Maximum number of distribution group members 100,000 members
Limit sending messages to large distribution groups 5,000 or more members
Maximum messages size for groups with 5,000-99,999 members 25MB
Maximum messages size for groups with 1,000 members 5MB

Mailbox Folder Limits

These limits are intended to limit mailboxes to known dimensions that can be supported in Exchange Online. The goal of these limits is to prevent an infinite number of mailbox items per folder, an infinite number of folders per mailbox, or an infinite number of public folders per Exchange Online organisation. For practical purposes, mailbox folder limits are in effect unlimited and sufficient to support most Exchange Online mailboxes and on-premises mailboxes that are migrated to Exchange Online.

The key folder limits are highlighted below, refer back to the Microsoft guidance for the full comprehensive view of all folder parameters:

Feature National N365 E3
Maximum number of messages per mailbox folder 1 million
Maximum number of subfolders per mailbox folder 10,000
Maximum folder hierarchy depth 300
Maximum number of public folders 500,000
Maximum number of subfolders per public folder 10,000

Message Limits

  • The following limits are applied to every email message in Exchange Online. These are in place to prevent large messages from blocking delivery of other messages and affecting service performance for all users.
Feature National N365 E3
Message size limit – Outlook 27 MB
Message size limit – OWA 27 MB
Message size limit – Outlook for Mac 27 MB
Message size limit – Outlook for iOS and Android 27 MB
Subject length limit 255 characters
File attachments limit 250 attachments
Multipart message limit 250 parts
Embedded message depth limit 30 embedded messages
Last Reviewed Date 3/9/2020
Updated on 25/07/2022

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