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 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.
  
  

Organizational Limits

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 Note: 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

Global Limits

Connector Limits

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 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	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

Server Limits

User Limits

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 Note: 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
  
  

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.