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Message size limits

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Applies to: Exchange Server 2013

You can apply limits to messages that move through the Microsoft Exchange Server 2013 organization. You can restrict the total size of a message or the size of the individual components of a message, such as the message header, the message attachments, and the number of recipients. You can apply limits globally for the whole Exchange organization, or specifically to a connector or user object.

As you plan the message size limits for your Exchange organization, consider the following questions:

  • What size limits should I impose on all incoming messages?
  • What size limits should I impose on all outgoing messages?
  • What is the mailbox quota for my Exchange organization?
  • How do the message size limits that I have chosen relate to the mailbox quota size?
  • Are there users in my Exchange organization who must send or receive messages that are larger than the specified allowed size?
  • Does my Exchange network topology include other messaging systems or distinctly separate business units that have different message size limits?

This topic provides guidance to help you answer these questions.

Types of message size limits

Following are the basic categories of the size limits available for individual messages:

  • Message header size limits: These limits apply to the total size of all message header fields that are present in a message. 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 character of text consumes 1 byte.

    Note

    Some third-party firewalls or proxy servers apply their own message header size limits. These third-party firewalls or proxy servers may have difficulty processing messages that contain attachment file names that are greater than 50 characters or attachment file names that contain non-US-ASCII characters.

  • Message size limits: These limits apply to the total size of a message, which includes the message header, the message body, and any attachments. Message size limits may be imposed on incoming messages or outgoing messages. For internal message flow, Exchange uses the custom X-MS-Exchange-Organization-OriginalSize: message header to record the original message size of the message as it enters the Exchange organization. Whenever the message is checked against the specified message size limits, the lower value of the current message size or the original message size header is used. The size of the message can change because of content conversion, encoding, and agent processing.

  • Attachment size limits: These limits apply to the maximum allowed size of a single attachment within a message. The message may contain many attachments that greatly increase the overall size of the message. However, an attachment size limit applies to the size of an individual attachment only.

  • Recipient limits: These limits apply to the total number of message recipients. When a message is first composed, the recipients exist in the To:, Cc:, and Bcc: header fields. When the message is submitted for delivery, the message recipients are converted into RCPT TO: entries in the message envelope. A distribution group is counted as a single recipient during message submission.

Scope of limits

Following are the basic categories for the scope of the limits available for individual messages:

  • Organizational limits: These limits apply to all Exchange 2013 Mailbox servers and Exchange 2010 and Exchange 2007 Hub Transport servers that exist in the organization. If you have an Edge Transport server installed in the perimeter network, the specified limits apply to the specific server.

  • Connector limits: These limits apply to any messages that use the specified Send connector, Receive connector, Delivery Agent connector, or Foreign connector for message delivery. Send connectors are defined in the Transport service on Mailbox servers and on Edge Transport servers. Receive connectors are defined in the Transport service on Mailbox servers, in the Front End Transport service on Client Access servers, and on Edge Transport servers.

  • Active Directory site links: The Transport service on Mailbox servers use Active Directory sites and the costs that are assigned to the Active Directory IP site links as one of the factors to determine the least-cost routing path between Mailbox servers in the organization. You can assign specific message size limits to the Active Directory site links in your organization.

  • Server limits: These limits apply to a specific Mailbox server or Edge Transport server. You can set the specified message limits independently on each Mailbox server or Edge Transport server.

    In Outlook Web App, the maximum HTTP request size limit setting on the Client Access servers also controls the size of messages that Outlook Web App users can send.

  • User limits: These limits apply to a specific user object, such as a mailbox, contact, distribution group, or public folder.

The following tables show the message limits, including information about how to configure the limits in the Exchange Management Shell or the Exchange Administrator Center (EAC).

Organizational limits

Size limit Default value Shell configuration EAC configuration
Maximum size for messages received 10 MB Cmdlet: Set-TransportConfig

Parameter: MaxReceiveSize		 Mail flow > Receive connectors > More options More Options Icon > Organization transport settings > Limits tab > Maximum receive message size
Maximum size for messages sent 10 MB Cmdlet: Set-TransportConfig

Parameter: MaxSendSize		 Mail flow > Receive connectors > More options More Options Icon > Organization transport settings > Limits tab > Maximum send message size
Maximum number of recipients per message 5000 Cmdlet: Set-TransportConfig

Parameter: MaxRecipientEnvelopeLimit		 Mail flow > Receive connectors > More options More Options Icon > Organization transport settings > Limits tab > Maximum number of recipients
Maximum attachment size in Transport rules that apply to all Mailbox servers in the organization Not configured Cmdlets: New-TransportRule, Set-TransportRule

Parameter: AttachmentSizeOver		 Mail flow > Rules > Add Add Icon or Edit Edit icon.

Use the predicate Apply this rule if > Any attachment > is greater than or equal to

Use the predicate Apply this rule if > The message > size is greater than or equal to
Maximum message size in Transport rules that apply to all Mailbox servers in the organization Cmdlets: New-TransportRule, Set-TransportRule

Parameter: MessageSizeOver		 Mail flow > Rules > Add Add Icon or Edit Edit icon.

Use the predicate Apply this rule if > The message > size is greater than or equal to

Connector limits

Size limit Default value Shell configuration EAC configuration
Maximum header size through a Receive connector 128 KB Cmdlets: New-ReceiveConnector, Set-ReceiveConnector

Parameter: MaxHeaderSize		 N/A
Maximum message size through a Receive connector

Note: The actual message size may be smaller due to message encoding and content conversion.		 Transport service on Mailbox servers

35 MB for the Default and Client Proxy Receive connectors

Front End Transport service on Client Access servers

36 MB for the Default Frontend and Outbound Proxy Frontend Receive connectors.

35 MB for the Client Frontend Receive connector.		 Cmdlets: New-ReceiveConnector, Set-ReceiveConnector

Parameter: MaxMessageSize		 Mail flow > Receive connectors > Edit Edit icon > General tab > Maximum receive message size
Maximum number of recipients per message through a Receive connector Transport service on Mailbox servers

5,000 for the Default Receive connector

200 for the Client Proxy Receive connector

Front End Transport service on Client Access servers

200 for the Default Frontend, Client Frontend, and Client Proxy Frontend Receive connectors.

Note: If the number of recipients is exceeded for an anonymous sender, the message is accepted for the first 200 recipients. Most SMTP messaging servers detect that a recipient limit is in effect. The SMTP messaging server continues to resend the message in groups of 200 recipients until the message is delivered to all recipients.		 Cmdlets: New-ReceiveConnector, Set-ReceiveConnector

Parameter: MaxRecipientsPerMessage		 N/A
Maximum message size through a Send connector 10 MB Cmdlets: New-SendConnector, Set-SendConnector

Parameter: MaxMessageSize		 Mail flow > Send connectors > Edit Edit icon > General tab > Maximum send message size
Maximum message size through an Active Directory site link Unlimited Cmdlet: Set-AdSiteLink

Parameter: MaxMessageSize		 N/A
Maximum message size through a delivery agent connector Unlimited Cmdlets: New-DeliveryAgentConnector, Set-DeliveryAgentConnector

Parameter: MaxMessageSize		 N/A
Maximum message size through a foreign connector Unlimited Cmdlet: Set-ForeignConnector Parameter: MaxMessageSize N/A

Server limits

Size limit Default value Shell configuration EAC configuration
Maximum header size for messages in the pickup directory 64 KB Cmdlet: Set-TransportService

Parameter: PickupDirectoryMaxHeaderSize		 N/A
Maximum number of recipients per message for messages in the pickup directory 100 Cmdlet: Set-TransportService

Parameter: PickupDirectoryMaxRecipientsPerMessage		 N/A
Client-specific maximum messages size limits for Outlook Web App, Exchange ActiveSync, and Exchange Web Services clients Outlook Web App: 35 MB

Exchange ActiveSync: 10 MB

Exchange Web Services: 64 MB

Note: These values are approximately 33% larger than the actual usable maximum message size because of the overhead that's associated with Base64 encoding.		 You configure these values in the appropriate web.config XML application configuration file on Client Access servers. For more information, see Configure client-specific message size limits. N/A

User limits

Size Limit Default value Shell configuration EAC configuration
Maximum message size that can be sent by this recipient Unlimited Cmdlets:

Set-DistributionGroup

Set-DynamicDistributionGroup

Set-Mailbox

Set-MailContact

Set-MailUser

Set-MailPublicFolder

Set-RemoteMailbox

Parameter: MaxSendSize		 For mailboxes:

Recipients > Mailboxes > Edit Edit icon > Mailbox features > Mail flow > Message size restrictions > View details > Sent messages

Note: This setting isn't configurable using the EAC for other recipient types.
Maximum message size that can be sent to this recipient Unlimited

For site mailbox provisioning policies: 36 MB		 Cmdlets:

Set-DistributionGroup

Set-DynamicDistributionGroup

Set-Mailbox

Set-MailContact

Set-MailUser

Set-MailPublicFolder

New-SiteMailboxProvisioningPolicy

Set-SiteMailboxProvisioningPolicy

Parameter: MaxReceiveSize		 For mailboxes:

Recipients > Mailboxes > Edit Edit icon > Mailbox features > Mail flow > Message size restrictions > View details > Received messages

Note: This setting isn't configurable using the EAC for other recipient types.
Maximum number of recipients per message sent by this recipient Unlimited Cmdlets:

Set-Mailbox, Set-MailUser

Parameter: RecipientLimits		 N/A

Order of precedence for message size limits

You can set different message size limits at different levels in the Exchange organization. As a message is routed through your Transport infrastructure, it may be subjected to several different message size restrictions. You should plan your message size restrictions in a way that makes sure that messages in the transport pipeline are rejected as early as possible if they violate message size limits. Generally speaking, you should set more restrictive limits at the points where messages enter your infrastructure. For example, any message size restrictions on your Receive connectors that receive messages from the Internet should be less than or equal to the message size restrictions you configure for your internal Exchange organization. It would be a waste of system resources for the Exchange server to accept and process a message from the Internet that would be rejected by the Transport service on your Mailbox servers. Make sure that your organization, server, and connector limits are configured in a way that minimizes any unnecessary processing of messages.

One exception to this approach is the user limits. User level limits take precedence over other message size restrictions. Therefore, you can configure a user to exceed the default message size limits for your organization. For example, you can allow a specific group of user mailboxes to send larger messages than the rest of the organization by configuring custom send and receive limits for those mailboxes.

The exceptions for the user limits only apply to message exchanges between authenticated users. If a message is sent to or received by a recipient on the Internet, the organizational limits will be applied. For example, assume that you have an organizational message size restriction of 10 MB, but you have configured the users in your marketing department to send and receive messages up to 50 MB. These users will be able to exchange large messages with each other, but they still won't be able to receive large messages from Internet users because such messages will be coming from unauthenticated senders.

Messages exempt from size limits

The following list shows the types of messages generated by a Mailbox server or an Edge Transport server and exempted from all message size limits:

  • System messages
  • Agent-generated message
  • Delivery status notification (DSN) messages
  • Journal report messages
  • Quarantined messages

However, these messages are still subject to the organizational value for maximum number of recipients in a message.