Address book limits
Address list limit: The maximum number of address lists that can be created in an Exchange Online or Exchange Server 2013 organization. This number includes the default address lists in Exchange Online, such as All Contacts and All Groups.
Offline 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: The maximum number of global address lists (GAL) that be created in an Exchange Online or Exchange Server 2013 organization.
Address book limits
Address book limits across standalone plans
Mailbox storage limits
The amount of mailbox storage available is determined by the mailbox type and the user's subscription license. Administrators can reduce maximum mailbox sizes per user or globally.
Storage limits across standalone plans
Exchange Online provides three kinds of notifications when a user's mailbox is nearing, or at, capacity:
Warning: 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 can't 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.
Capacity alerts across standalone plans
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.
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.
Mailbox folder limits
Mailbox folder limits across standalone plans
The following limits are applied to every email message.
Message size limit: 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. To learn more, see
Message header size limit: Specifies the maximum size of all message header fields in a message. The current limit is 256 KB. If the total size of all message headers exceeds 256 KB, Exchange Online will reject the message with error "552 5.3.4 Header size exceeds fixed maximum size." The size of the message body or attachments isn't considered. Because the header fields are plain text, the size of the header is determined by the number of characters in each header field and by the total number of header fields. Each text character consumes 1 byte.
Subject length limit: The maximum number of text characters allowed in the subject line of an email message.
File attachments limit: The maximum number of file attachments allowed in an email message. Even if the total size of all the file attachments doesn't violate the message size limit, there is still a limit on how many attachments are allowed in the message. This limit is controlled by the multipart message limit.
File attachment size limit: The maximum file size of a single attachment.
Multipart message limit: The maximum number of message body parts that are allowed in a MIME multipart message. This limit also controls the maximum number of file attachments that are allowed in a message.
Embedded message depth limit: The maximum number of forwarded email messages that are allowed in an email message.
Message limits across standalone options
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.
Receiving limits apply to the number of messages that an Exchange Online user, group, or public folder can receive.
- Receiving limit: This limit applies to the number of messages per hour from any and all sources. This includes messages from internal senders, messages from the internet, and messages from on-premises servers. When the receiving limit has been exceeded on a mailbox, messages sent to the mailbox will be returned to the sender in a non-delivery report (also known as an NDR or bounce message) stating that the mailbox has exceeded the maximum delivery threshold. After an hour, the limit will refresh and the mailbox will be able to receive messages.
- Sender-recipient pair limit: This limit applies to the number of messages per hour from a single sender. This is set to a ratio of the overall receiving limit to protect against single-sender mail storms.
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.
Recipient proxy address limit: The recipient proxy address limit is the maximum number of aliases (email addresses) that a recipient mailbox can have.
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.
Sending limits across standalone options
Reporting and message trace limits
For reporting and message trace limits, see the "Reporting and message trace data availability and latency" section in Reporting and message trace in Exchange Online Protection.
These limits control the length of time that items in specific folders in the Inbox can be accessed.
Deleted Items folder retention period: The maximum number of days that items can remain in the Deleted Items folder before they're automatically removed.
Retention period for items removed from the Deleted Items folder: The maximum numbers of days that items removed from the Deleted Items folder are retained before they're permanently deleted.
Junk Email folder retention period: The maximum number of days that items can remain in the Junk Email folder before they're automatically removed.
Retention limits across standalone options
Distribution group limits
These limits apply to distribution groups in your organization's 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).
Distribution group limits
Distribution group limits across standalone options
Journal, transport, and inbox rule limits
The following list includes limits that apply to journal rules, transport rules (also known as organization-wide rules), and limits that apply to inbox rules. Inbox rules are set up by individual users and applied to messages sent and received by the individual user's mailbox.
Maximum number of journal rules: The maximum number of journal rules that can exist in the organization.
Maximum number of transport rules: The maximum number of rules that can exist in the organization.
Maximum size of an individual transport rule: The maximum number of characters that can be used in a single transport rule. The characters are used in the conditions, exceptions, and actions.
Character limit for all regular expressions used in all transport rules: The total number of characters used including all the regular expressions in all the transport rule conditions and exceptions in the organization. You can have a few rules that use long and complex regular expressions, or you can have many rules that use simple regular expressions.
Scanning limits for attachment content: The transport rule conditions let you examine the content of message attachments, but only the first 1 MB of the text extracted from an attachment is inspected. This 1 MB limit refers to the text extracted from the attachment, not to the file size of the attachment. For example, a 2 MB file may contain less than 1 MB of text, so all of the text would be inspected.
Maximum number of recipients added to a message by all transport rules: When a message is acted on by different transport rules, only a finite number of recipients can be added to the message. After the limit is reached, any remaining recipients aren't added to the message. Also, distribution groups can't be added to a message by a transport rule.
Forwardee limit: The maximum number of recipients that can be configured for an inbox or transport rule with a redirecting action. If a rule is configured to redirect a message to more than this number of recipients, the rule won't be applied and any message that satisfies the rule condition won't be redirected to any of the recipients listed in the rule.
Number of times a message is redirected: The number of times a message will be redirected, forwarded, or replied to automatically based on Inbox rules. For example, User A has an Inbox rule that redirects messages to User B, based on the sender. User B has an Inbox rule that forwards messages to User C based on keywords in the subject line. If a message satisfies both of these conditions, the message is only sent to User B; it's not forwarded to User C because only one redirection is allowed. In this case, the message is dropped without sending a non-delivery report (NDR) to User B indicating that the message wasn't delivered to User C. We make use of the X-MS-Exchange-Inbox-Rules-Loop header to determine the number of times that a message was redirected. This header remains also across Exchange organizational boundaries.
Number of times a message is redirected by transport rules: The number of times a message will be redirected based on transport rules. For example, Exchange organization Tailspin Toys has a transport rule to redirect every message that was sent to User A to User B, which is located in Exchange organization Contoso. Within Exchange organization Contoso, there is a transport rule in place to redirect every message that was sent to User B to User C, which is located in Exchange organization A. Datum Corporation. In this case, the message is dropped and a non-delivery report (NDR) with status code and reject message 550 5.7.128 TRANSPORT.RULES.RejectMessage; Transport rules loop count exceeded and message rejected is sent to User A. We make use of the X-MS-Exchange-Transport-Rules-Loop header to determine the number of times that a message was redirected by transport rules. This header remains also across Exchange organizational boundaries.
Journal, transport, and inbox rule limits
Journal, transport, and inbox rule limits across standalone options
These limits control the moderation settings that are used for message approval applied to distribution groups and transport rules.
Maximum size of the arbitration mailbox: If the arbitration mailbox exceeds this limit, messages that require moderation are returned to the sender in a non-delivery report (NDR).
Maximum number of moderators: The maximum number of moderators that you can assign to a single moderated distribution group or that can be added to a message using a single transport rule. Note that you can't specify a distribution group as a moderator.
Expiration for messages waiting for moderation: By default, a message waiting for moderation expires after two days, and admins cannot extend this interval. However, the processing of expired moderated messages runs every seven days. This means that a moderated message can expire at any time between two and nine days.
Maximum rate for expired moderation notification messages: This limit sets the maximum number of notification messages for expired moderated messages in a one-hour period. This limit is placed on each mailbox database in the datacenter.
During periods of heavy usage, some senders may not receive notification messages for moderated messages that have expired. However, these notifications are still discoverable using delivery reports.
Moderation limits across standalone options
Exchange ActiveSync limits
The following limits apply to Microsoft Exchange ActiveSync, a client protocol that synchronizes mailbox data between mobile devices and Exchange.
Exchange ActiveSync device limit: The maximum number of Exchange ActiveSync devices per mailbox.
Exchange ActiveSync device deletion limit: The maximum number of Exchange ActiveSync devices that an Exchange administrator can delete in a single month.