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Understanding Message Size Limits

Exchange 2010
 

Applies to: Exchange Server 2010 SP3, Exchange Server 2010 SP2

Topic Last Modified: 2013-07-01

You can apply message size limits to individual messages that move through the Microsoft Exchange Server 2010 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 2010 organization, or specifically for a particular connector or user object.

As you plan the message size limits for your Exchange 2010 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?

  • Does my Exchange 2010 organization have a mailbox quota?

  • How do the message size limits that I have chosen relate to the mailbox quota size?

  • Are there users in my Exchange 2010 organization who must send or receive messages that are larger than the specified allowed size?

  • Does my Exchange 2010 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.

Looking for management tasks related to Transport servers? See Managing Transport Servers.

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.

    noteNote:
    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 2010 uses the custom X-MS-Exchange-Organization-OriginalSize: message header to record the original message size of the message as it enters the Exchange 2010 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.

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

  • Organizational limits   These limits apply to all Exchange 2010 and Exchange 2007 servers that exist in the organization. The specified message limits apply to all Exchange 2010 and Exchange 2007 servers that have the Hub Transport server role installed. On an Edge Transport server, the specified limits apply to the specific server.

  • Global limits   Global limits are used when Exchange Server 2003 servers coexist with your Exchange 2010 deployment. The global limits are stored in a different location in Active Directory than the organizational limits and are primarily used by Exchange Server 2003 servers. In an environment where you have both Exchange 2010 and Exchange Server 2003 servers in the same organization, changes that you make to the organizational limits are automatically copied to the corresponding global limits. In Exchange 2010, you can modify the organizational limits by using the Set-TransportConfig cmdlet in the Exchange Management Shell, or by configuring the Hub Transport server organization configuration properties in the Exchange Management Console.

  • 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. Connectors are defined on Hub Transport servers or Edge Transport servers.

  • Active Directory site links   Hub Transport servers use Active Directory sites and the costs that are assigned to the Active Directory IP site links to determine the least-cost routing path from each Hub Transport server in the organization to every other Hub Transport server in the organization. You can assign specific message size limits to the Active Directory site links in your organization. For example, you may want to apply a lower message size limit to an Active Directory site link that represents a connection to a remote office with a low-bandwidth connection. Any messages that exceed the maximum message size limit on any Active Directory site link included in the least-cost routing path won't be delivered and will generate a delivery status notification (DSN) with a value of 5.3.4. For more information about message routing in Exchange 2010, see Planning to Use Active Directory Sites for Routing Mail.

  • Routing group connectors   A routing group connector is used to send and receive messages between Exchange 2010 Hub Transport servers and Exchange Server 2003 bridgehead servers when the organization is running more than one version of Microsoft Exchange. Any messages that exceed the maximum message size limit on any routing group connector won't be delivered. They will generate a DSN with the value of 5.3.4. For more information about routing group connectors, see Upgrade from Exchange 2003 Transport.

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

    If you are using 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. For more information, see Configure Maximum Message Size in Outlook Web App.

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

The following table shows the organizational limits, including information about how to configure the limits in the Exchange Management Shell or the Exchange Management Console (EMC).

Organizational limits

Size limit Default value in Exchange 2010 Shell configuration EMC configuration

Maximum size for messages received

10 MB

Cmdlet: Set-TransportConfig

Parameter: MaxReceiveSize

Organization Configuration > Hub Transport > Global Settings > Transport Settings > General tab

Maximum size for messages sent

10 MB

Cmdlet: Set-TransportConfig

Parameter: MaxSendSize

Organization Configuration > Hub Transport > Global Settings > Transport Settings > General tab

Maximum number of recipients per message

noteNote:
When a message is first processed by a Hub Transport server, an X-header named X-MS-Exchange-Organization-OriginalSize: is inserted into the message header. Any Hub Transport servers that are involved in the future delivery of the message will use this value for the message size. Conversion encoding and agent processing can increase the size of the message as it flows through the Exchange organization.

5000

Cmdlet: Set-TransportConfig

Parameter: MaxRecipientEnvelopeLimit

Organization Configuration > Hub Transport > Global Settings > Transport Settings > General tab

Maximum attachment size in Transport rules that apply to all Hub Transport servers in the organization

Not configured

Cmdlets: New-TransportRule, Set-TransportRule

Parameter: AttachmentSizeOver

Organization Configuration > Hub Transport > Transport Rules

New Transport Rule wizard or Edit Transport Rule wizard

The following table shows the global limits, including information about where to configure the limits in Exchange System Manager in Exchange Server 2003.

Global limits

Size limit Default value Exchange System Manager configuration

Maximum size for messages received

10240 KB (10 MB)

  • delivContLength in Active Directory

  • Incoming message size in Exchange System Manager Global Settings

Maximum size for messages sent

10240 KB (10 MB)

  • submissionContLength in Active Directory

  • Outgoing message size in Exchange System Manager Global Settings

Maximum number of recipients per message

5000

  • msExchRecipLimit in Active Directory

  • Maximum number of recipients in Exchange System Manager Global Settings

The following table shows the connector limits, including information about how to configure the limits in the Exchange Management Shell or the Exchange Management Console (EMC).

Connector limits

Size limit Default value Shell configuration EMC configuration

Maximum header size through a Receive connector

64 KB

Cmdlets: New-ReceiveConnector, Set-ReceiveConnector

Parameter: MaxHeaderSize

N/A

Maximum message size through a Receive connector

10 MB

Cmdlets: New-ReceiveConnector, Set-ReceiveConnector

Parameter: MaxMessageSize

Server Configuration > Hub Transport > Receive Connectors > Receive Connector properties > General tab

Edge Transport > Receive Connectors > Receive Connector properties > General tab

Maximum number of recipients per message through a Receive connector

200 for the Default Client Receive connector

5,000 for the Default Receive connector on Hub Transport servers

200 for the Default Receive connector on Edge Transport servers

noteNote:
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

Organization Configuration > Hub Transport > Send Connectors > Send Connector properties > General tab

Edge Transport > Send Connectors > Send Connector properties > General tab

Maximum message size through an Active Directory site link

Unlimited

Cmdlet: Set-AdSiteLink

Parameter: MaxMessageSize

N/A

Maximum message size through a routing group connector

Unlimited

Cmdlet: Set-RoutingGroupConnector

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

The following table shows the server limits, including information about how to configure the limits in the Exchange Management Shell or the Exchange Management Console (EMC).

Server limits

Size limit Default value Shell configuration EMC configuration

Transport rule on an Edge Transport server that only applies to the specific server

Not configured

Cmdlets: New-TransportRule, Set-TransportRule

Parameter: AttachmentSizeOver

Edge Transport > Transport Rules

New Transport Rule wizard or Edit Transport Rule wizard

Maximum header size for messages in the pickup directory

64 KB

Cmdlet: Set-TransportServer

Parameter: PickupDirectoryMaxHeaderSize

Not applicable

Maximum number of recipients per message for messages in the pickup directory

100

Cmdlet: Set-TransportServer

Parameter: PickupDirectoryMaxRecipientsPerMessage

Not applicable

In addition, you can configure the maximum HTTP request length on your Client Access servers that service Microsoft Office Outlook Web App clients. The value configured for this setting will also affect the message size users can submit. For example, if you set this value lower than other message size limits in your organization, your users will not be able to send large messages using Outlook Web App even though they can send the same message using Outlook.

You can configure this setting by modifying the maxRequestLength parameter in the web.config file on your Client Access servers. By default, this file is located in the <Exchange install directory>\V14\ClientAccess\Owa folder. The default value is 30000 KB.

The following table shows message size limits that you can configure at the recipient level, including information about how to configure the limits in the Exchange Management Shell or the Exchange Management Console (EMC).

User limits

Size Limit Default value Shell configuration EMC 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

Parameter: MaxSendSize

For mailboxes:

Recipient Configuration > Mailbox Properties > Mail Flow Settings tab

For mail public folders:

Public Folder Management Console > Public Folder properties > Mail Flow Settings tab

noteNote:
This setting isn't configurable using the EMC for other recipient types.

Maximum message size that can be sent to this recipient

Unlimited

Cmdlets:

Set-DistributionGroup

Set-DynamicDistributionGroup

Set-Mailbox

Set-MailContact

Set-MailUser

Set-MailPublicFolder

Parameter: MaxReceiveSize

For all recipient types except Mail Public Folders:

Recipient Configuration > Recipient Properties > Mail Flow Settings tab

For Mail Public Folders:

Public Folder Management Console > Public Folder properties > Mail Flow Settings tab

Maximum number of recipients per message sent by this recipient

Unlimited

Cmdlets:

Set-Mailbox, Set-MailUser

Parameter: RecipientLimits

Cmdlet: Set-MailUser

Parameter: MaxRecipientsPerMessage

N/A

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 Edge server 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 Edge Transport server to accept and process a message from the Internet that would be rejected by your Hub Transport 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 users.

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.

The following list shows the types of messages generated by a Hub Transport 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

importantImportant:
Even though they are generated by the system, non-delivery reports (NDRs) are not exempt from message size limits.

However, these messages are still subject to the organizational value for maximum number of recipients in a message. This value is set by the MaxRecipientEnvelopeLimit parameter that you can configure by using the Set-TransportConfig cmdlet in the Shell.

The primary difference in message size limits between Exchange Server 2003 and Exchange Server 2010 is in the handling of recipient limits. Exchange 2003 treats each member of an expanded distribution list as one recipient. Exchange 2010 treats a distribution group as one recipient. This change was implemented to avoid the partial message delivery scenarios that may occur in Exchange 2003.

Partial message delivery occurs in Exchange 2003 if the number of individual recipients and the recipients that are contained within the distribution list exceeds the specified recipient limit. The total number of message recipients isn't known until after distribution list expansion. Message delivery occurs as the distribution list is expanded until the number of recipients reaches the specified limit. The remaining recipients do not receive the message, but at least the sender receives a non-delivery report (NDR) for each unsuccessful delivery. However, if delivery failure reporting is disabled for the distribution list, the remaining recipients wouldn't receive the message, and the sender would not know who did not receive the message.

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