Customers, which are planning to migrate to Exchange online, asks mostly the same questions.
How much do it costs to run Exchange online?
How the migration has to be made?
Which DNS settings has to be made?
How about the Mail domains?
Moreover, many more…
However, also often asked questions are about the limits in Exchange online. Here we have significant changes to standard on-premises Exchange environments.
If we talk about limits by Exchange online, we can take them in six Main categories:
- Address book limits
- Mailbox storage limits
- Capacity alerts
- Mailbox folder limits
- Message limits
- Receiving and sending limits
In this article, we will have a look about the limits and additional information to them. The limits applied to a Microsoft Office 365 organization may differ depending on how long the organization has been enrolled in the service. When a limit is changed in the Microsoft data centers, it can take some time to apply the change to all existing customers.
Address book limits
For the start we will have a look on the address book and global address lists (GAL) limits. Here we talk about:
- Address list limits
- The maximum number of address lists that can be created in an Exchange Online or Exchange on-premises (2013, 2016 or 2019) organization. This number includes the default address lists in Exchange Online, such as All Contacts and All Groups.
- Offiline address book limit
- The maximum number of offline address books (OAB) that can be created in an Exchange Online or Exchange Server 2013 organization.
- Address book policies limit
- The maximum number of address book policies (ABP) that be created in an Exchange Online or Exchange Server 2013 organization.
- Global address lists (GAL)
- The maximum number of global address lists (GAL) that be created in an Exchange Online or Exchange Server 2013 organization.
Office 365 has some several plans which starts from Office 365 Business Essentials till Office 365 Enterprise F1.
However, if we have a look on the address book limits across all Office 365 options, I can tell you they are all the same and there are no different.
Shown in a table it looks like this:
| Address list limit | 1000 |
| Office address book (OAB) limit | 250 |
| Address book policies (ABP) limit | 250 |
| Global address lists limit | 250 |
Note: Analog to the different Office 365 potions, the same limits matches to the standalone plans as well.
Here we talk about Exchange Online Plan 1, Exchange Online Plan 2 and Exchange Online Kiosk.
Mailbox storage limits
If we have a look on our on-premises Exchange environment in the past, it was like this: We had an amount of storage for our Exchange environment, on this storage we had created mailbox databases and, usually, we crated mailboxes with approximately 4 GB storage limits for each mailbox.
By using Exchange online we don’t need to think about limited storage capacities. Of course we also have limits for the mailbox storage. However the limit depends about the plan we are using. This, because by buying an Exchange online license we don’t buy just the mail service and features, we also buy with it the storage for the mailboxes.
The limits for that are divided in tree main categories:
- Cat 1 (Business Essentials, Business Premium Enterprise E1)
- Cat 2 (Enterprise E3, Enterprise E5)
- Cat 3 (Enterprise F1)
By setting the Office 365 options in this tree categories, our limit table is looking like this:
| Feature | Cat 1 | Cat 2 | Cat 3 |
| User Mailbox | 50 GB | 100 GB | 2 GB |
| Archive Mailboxes | 50 GB | Unlimited | Not available |
| Shared Mailboxes | 50 GB | 50 GB | 50 GB |
| Resource Mailboxes | 50 GB | 50 GB | 50 GB |
| Site Mailboxes | 50 GB | 50 GB | Not available |
| Public Folder Mailboxes | 50 GB | 100 GB | Not available |
| Group Mailboxes | 50 GB | 50 GB | 50 GB |
However, let us also have a look about the standalone plans (Exchange online):
| Feature | EO Plan 1 | EO Plan 2 | EO Kisosk |
| User Mailbox | 50 GB | 100 GB | 2 GB |
| Archive Mailboxes | 50 GB | Unlimited | Not available |
| Shared Mailboxes | 50 GB | 50 GB | 50 GB |
| Resource Mailboxes | 50 GB | 50 GB | 50 GB |
| Public Folder Mailboxes | 50 GB | 100 GB | Not available |
| Group Mailboxes | 50 GB | 50 GB | 50 GB |
Note: A shared mailbox is not designed for direct logon. The user account for the shared mailbox itself should stay in a Disabled (or “disconnected”) state.
Capacity alerts
The next category we are looking for are the capacity alerts. Exchange Online provides three kinds of notifications when a user’s mailbox is nearing, or at, capacity:
Microsoft Exchange distinguishes between the following topics:
- Warnings
- The user receives an email warning that the mailbox is approaching the maximum size limit. This warning is intended to encourage users to delete unwanted mail.
- Prohibit Send
- The user receives a prohibit-send notification email when the mailbox size limit is reached. The user cannot send new messages until enough email is deleted to bring the mailbox below the size limit.
- Prohibit Send/Receive
- Exchange Online rejects any incoming mail when the mailbox size limit is reached, and sends a non-delivery report (NDR) to the sender. The sender has the option to try resending the mail later. To receive messages again, the user must delete email until the mailbox is below the size limit.
Therefore, if we have here to a more detailed view across the Office 365 options, it looks like in the table below:
| Feature | Cat 1 | Cat 2 | Cat 3 |
| Warning | 49 GB | 98 GB | 1.96 GB |
| Prohibit Send | 49.5 GB | 99 GB | 1.98 GB |
| Prohibit Send/Receive | 50 GB | 100 GB | 2 GB |
As well, we have a look on the standalone plans below:
| Feature | EO Plan 1 | EO Plan 2 | EO Kiosk |
| Warning | 49 GB | 98 GB | 1.96 GB |
| Prohibit Send | 49.5 GB | 99 GB | 1.98 GB |
| Prohibit Send/Receive | 50 GB | 100 GB | 2 GB |
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 organization. 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.
However, if we split all features down, we talk about the following folders:
- Maximum number of messages per mailbox folder
- Specifies the maximum number of messages for a mailbox folder. New messages can’t be delivered or saved in a folder when this limit is reached.
- Warning for number of messages per mailbox folder
- Specifies the number of messages that a mailbox folder can hold before Exchange Online sends a warning message to the mailbox owner. When this quota is reached, warning messages are sent once a day.
- Maximum number of messages per folder in the Recoverable Items folder
- Specifies the maximum number of messages that can be contained in each folder in the Recoverable Items folder. When a folder exceeds this limit, it can’t store new messages. For example, if the Deletions folder in the Recoverable Items folder has exceeded the message count limit and the mailbox owner attempts to permanently delete items from their mailbox, the deletion will fail.
- Warning for number of messages per folder in the Recoverable Items folder
- Specifies the number of messages that each folder in the Recoverable Items folder can hold before Exchange Online logs an event to the application event log.
- Maximum number of subfolders per mailbox folder
- Specifies the maximum number of subfolders that can be created in a mailbox folder. The mailbox owner won’t be able to create a new subfolder when this limit is reached.
- Warning for number of subfolders per mailbox folder
- Specifies the number of subfolders that can be created in a mailbox folder before Exchange Online sends a warning message to the mailbox owner. When this quota is reached, warning messages are sent once a day.
- Maximum folder hierarchy depth
- Specifies the maximum number of levels in the folder hierarchy of a mailbox. The mailbox owner won’t be able to create another level in the folder hierarchy of the mailbox folder when this limit is reached.
- Warning for folder hierarchy depth
- Specifies the number of levels in the folder hierarchy of a mailbox folder that can be created before Exchange Online sends a warning message to the mailbox owner. When this quota is reached, warning messages are sent once a day.
- Maximum number of public folders
- Specifies the maximum number of public folders in the complete public folder hierarchy. When this limit is reached, existing public folders must be deleted before new public folders can be created.
- Maximum number of subfolders per public folder
- Specifies the maximum number of subfolders that can be created in a public folder. New subfolders can’t be created in a public folder when this limit is reached.
- Warning for number of subfolders per public folder
- Specifies the number of subfolders that can be created in a public folder before Exchange Online sends a warning message to the folder owner. If no owner exists, warning messages are sent to users with Owner permissions. When this quota is reached, warning messages are sent once a day.
As well, we will have here a look to how it looks with the different Office 365 options:
| Feature | Cat 1 | Cat 2 | Cat 3 |
| Maximum number of messages per mailbox folder | 1 million | 1 million | 1 million |
| Warning for number of messages per mailbox folder | 900’000 | 900’000 | 900’000 |
| Maximum number of messages per folder in the Recoverable Items folder | 3 million | 3 million | 3 million |
| Storage quota for Recoverable Items folder in primary mailbox (not on hold) | 30 GB | 30 GB | 30 GB |
| Storage quota for Recoverable Items folder in primary mailbox (on hold) | 100 GB | 100 GB | 100 GB |
| Storage quota for Recoverable Items folder in archive mailbox (not on hold) | 30 GB | Unlimited | 30 GB |
| Storage quota for Recoverable Items folder in archive mailbox (on hold) | 100 GB | Unlimited | 100 GB |
| Warning for number of messages per folder in the Recoverable Items folder | 2.75 million | 2.75 million | 2.75 million |
| Maximum number of subfolders per mailbox folder | 10’000 | 10’000 | 10’000 |
| Warning for number of subfolders per mailbox folder | 9000 | 9000 | 9000 |
| Maximum folder hierarchy depth | 300 | 300 | 300 |
| Warning for folder hierarchy depth | 250 | 250 | 250 |
| Maximum number of public folders | 500’000 | 500’000 | Not available |
| Maximum number of subfolders per public folder | 10’000 | 10’000 | Not available |
| Warning for number of subfolders per public folder | 9000 | 9000 | Not available |
As well also a view on the Mailbox folder limits across the standalone plans:
| Feature | EO Plan 1 | EO Plan 2 | EO Kiosk |
| Maximum number of messages per mailbox folder | 1 million | 1 million | 1 million |
| Warning for number of messages per mailbox folder | 900’000 | 900’000 | 900’000 |
| Maximum number of messages per folder in the Recoverable Items folder | 3 million | 3 million | 3 million |
| Storage quota for Recoverable Items folder in primary mailbox (not on hold) | 30 GB | 30 GB | 30 GB |
| Storage quota for Recoverable Items folder in primary mailbox (on hold) | 100 GB | 100 GB | 100 GB |
| Storage quota for Recoverable Items folder in archive mailbox (not on hold) | 30 GB | 30 GB | 30 GB |
| Storage quota for Recoverable Items folder in archive mailbox (on hold) | 100 GB | Unlimited | 100 GB |
| Warning for number of messages per folder in the Recoverable Items folder | 2.75 million | 2.75 million | 2.75 million |
| Maximum number of subfolders per mailbox folder | 1000 | 1000 | 1000 |
| Warning for number of subfolders per mailbox folder | 900 | 900 | 900 |
| Maximum folder hierarchy depth | 300 | 300 | 300 |
| Warning for folder hierarchy depth | 250 | 250 | 250 |
| Maximum number of public folders | 100’000 | 100’000 | Not available |
| Maximum number of subfolders per public folder | 1000 | 1000 | Not available |
| Warning for number of subfolders per public folder | 900 | 900 | Not available |
Message limits
Another very important limitation we need to know is the limits of the messages itself. This limitation is one of the first, our users could reach out by working with Exchange Online.
Message size limits are necessary to prevent large messages from blocking delivery of other messages and affecting service performance for all users. These limits include attachments, and apply organization-wide to all messages (inbound, outbound, and internal). Messages larger than this limit will not be delivered, and the sender will receive a non-delivery report (NDR). While message size limits can be configured up, down, or on a per-user basis, administrators can also create transport rules to limit the maximum size of any individual attachment.
Note: Particular email clients may have lower message size limits or may limit the size of an individual file attachment to a value that is less than the Exchange Online message size limit.
Other limits which belongs to this chapter are the Subject length limit, the File attachments limit, and File attachment size limit.
As well the Multipart message for the maximum number of message body parts that are allowed in a MIME multipart message and last but not least the Embedded message depth limit for the maximum number of forwarded email messages that are allowed in an email message.
One more time I have listed the limits of the features in the table below:
| Feature | Cat 1 | Cat 2 | Cat 3 |
| Message size limit – Outlook | 150 MB | 150 MB | 150 MB |
| Message size limit – OWA | 112 MB | 112 MB | 112 MB |
| Message size limit – Outlook for Mac | 150 MB | 150 MB | 150 MB |
| Message size limit – migration | 150 MB | 150 MB | 150 MB |
| Size limit for encrypted messages (for subscribers using Office 365 Message Encryption with new capabilities) | 150 MB | 150 MB | 150 MB |
| Size limit for encrypted messages (for subscribers using Office 365 Message Encryption legacy version) | 25 MB | 25 MB | 25 MB |
| Subject length limit | 255 characters | 255 characters | 255 characters |
| File attachments limit | 250 attachments | 250 attachments | 250 attachments |
| File attachment size limit – Outlook | 150 MB | 150 MB | 150 MB |
| File attachment size limit – OWA | 35 MB | 35 MB | 35 MB |
| File attachment size limit – Outlook for Mac | 150 MB | 150 MB | 150 MB |
| Multipart message limit | 250 parts | 250 parts | 250 parts |
| Embedded message depth limit | 30 embedded messages | 30 embedded messages | 30 embedded messages |
As we can see, the limits are all the same, so it doesn’t matter which plan we are running. However, the same we can see below about the standalone Exchange online plans.The only different we see by the Exchange Online Kiosk:
| Feature | EO Plan 1 | EO Plan 2 | EO Kiosk |
| Message size limit – Outlook | 150 MB | 150 MB | 150 MB |
| Message size limit – OWA | 112 MB | 112 MB | 150 MB |
| Message size limit – Outlook for Mac | 150 MB | 150 MB | |
| Message size limit – migration | 150 MB | 150 MB | 150 MB |
| Size limit for encrypted messages (for subscribers using Office 365 Message Encryption with new capabilities) | 150 MB | 150 MB | 150 MB |
| Size limit for encrypted messages (for subscribers using Office 365 Message Encryption legacy version) | 25 MB | 25 MB | 25 MB |
| Subject length limit | 255 characters | 255 characters | 255 characters |
| File attachments limit | 250 attachments | 250 attachments | 250 attachments |
| File attachment size limit – Outlook | 150 MB | 150 MB | 150 MB |
| File attachment size limit – OWA | 35 MB | 35 MB | 35 MB |
| File attachment size limit – Outlook for Mac | 150 MB | 150 MB | 35 MB |
| Multipart message limit | 250 parts | 250 parts | 250 parts |
| Embedded message depth limit | 30 embedded messages | 30 embedded messages | 30 embedded messages |
Receiving and sending limits
Receiving and sending limits are applied to combat spam and mass-mailing worms or viruses. These limits help to protect the health of our systems and keep our users safe.
Note: For distribution groups stored in an organization’s address book, the group is counted as one recipient. For distribution groups stored in the Contacts folder of a mailbox, the members of the group are counted individually.
If we talk about Recipient rate limits, we 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.
By talking about the Recipient limits, it is about the maximum number of recipients allowed in the To:, Cc:, and Bcc: fields for a single email message.
Last but not least there are also Message rate limits. 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 overconsumption 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.
This limitations are for all Office 365 plans and standalone options the same. There are no differences between them. So the Table below is for all the same:
| Feature | Limits |
| Recipient rate limit | 10’000 recipient per day |
| Recipient limit | 500 recipients |
| Recipient proxy address limit | 400 |
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