Exchange 2000 Server and Exchange Server 2003 Message Restrictions
Topic Last Modified: 2006-01-12
By Kent Tilger
This article discusses message restrictions for Microsoft® Exchange Server 2003 and Exchange 2000 Server.
Consider the following definitions:
Max Submission Content Length (SubmissionContLength) is referred to as the Sending Size Limit.
Max Delivery Content Length (DelivContLength) is referred to as the Receive Size Limit.
Organization Wide Limits are synonymous with Global Limits.
Per-user configuration for (SubmissionContLength) overrides globally defined (SubmissionContLength) settings.
Per-user configuration for (DelivContLength) takes precedence over globally defined (DelivContLength) settings, but in later builds of the product, the message will not get to categorization if the (SubmissionContLength) check fails during submission.
Per-user configuration for (SubmissionContLength) does not override Simple Mail Transfer Protocol (SMTP) virtual server limits.
Per-user configuration for (SubmissionContLength) does not override connector defined limits.
Consider the following examples:
A user with a per-user Sending Limit value that is higher than the globally defined Send Limit can send a message larger than the globally defined limit up to the Sending Size Limit defined on the user object in the Active Directory® directory service.
Note: Be aware of inter-routing group message size increases when using SMTP between routing groups, which is resolved in Microsoft Exchange Server 2003 native mode.
A recipient with a per-user Receive Limit value that exceeds the globally defined Receive Limit can receive a message up to the size defined on the recipient for the Receive Limit.
Note: The previous examples assume a non-MAPI submission restriction check where Sending Size Limit values are not exceeded on submission.
The following is an overview of how Max Submission and Max Delivery size restriction evaluation works in a flowchart format.
Size restrictions are enforced at the following levels:
Message submission (MAPI clients)
If a protocol level restriction is configured on the virtual server properties, any message coming into the system with a message size that exceeds the configured size will be rejected at the protocol level by SMTP before making it to transport.
For MAPI client submission on Exchange Server 2003 and Exchange 2000 Server, the Max Submission Content Length or Sending Size Limit is evaluated on submit, so a message that is larger than either the per-user or globally defined Send Limit is rejected prior to submission.
In both previous examples, the message would be rejected prior to considering the Receive Size Limit on a recipient.
For messages that are submitted through SMTP, MTA, X.400, or another legacy connector, the Sending Size Limit and Receive Size Limit restrictions are evaluated during categorization.
For Sending Size Limit, the per-user limits on the Active Directory user object take precedence over globally defined Sending Size Limit values.
After the Max Submission Content Length restrictions have been evaluated, the Max Delivery Content Length (DelivContLength) is evaluated.
For Receive Size Limit, the per-user limits (Active Directory) take precedence over globally defined Receive Size Limit values.
If any message has a size that exceeds the globally defined Receive Size Limit and the recipient does not override the globally defined value with an explicit per-user defined Receiving Size value, the message will generate a 5.2.3 delivery status notification:
This message is larger than the current system limit or the recipient's mailbox is full.
Create a shorter message body or remove attachments and try sending it again. <Server.Domain.com #5.2.3>
The categorizer applies the globally defined Receive Size Limit restriction to local and remote deliveries. Therefore, a message sent to a non-organization (Internet) recipient that exceeds the globally defined Receive Size Limit will generate a 5.2.3 delivery status notification:
Receive Size Limits cannot be overridden by explicit Sending Size Limit values defined Per-User, but explicit Receive Size Limit values do take precedence over globally defined Receive Size values.
If the message is not for local delivery, routing chooses the most optimal path, considering variables like cost, message type, and restrictions, and locates the next server for a message to make the next hop to, and gives this next-hop server name to Advanced Queuing.
SMTP virtual server limits and connector limits apply at each stage in routing.
The same restriction checking logic is then repeated on each server that handles the message in transit to the final destination.
Consider the following potential issues:
Enforce Sending Size restrictions consistently on message submission as opposed to trying to enforce later during categorization.
For information about a fix to this issue, see Microsoft Knowledge Base article 836738, "E-mail messages that are larger than the "Sending message size" delivery option are delivered in Exchange 2000 Server." Global Sending Size restrictions were inefficient prior to the fix, because the submission limit was not evaluated until categorization. The potential flaw in this logic was that unless there was an explicit Sending Limit set, some users could (knowingly or unknowingly) submit very large messages into the system. Because the message was not evaluated until categorization, the messages would enter the system, only to be rejected later in transport with a 5.2.3 delivery status notification.
The new behavior for MAPI clients is that the Sending Limit restriction checking is done during message submission (as opposed to categorization). If the message exceeds the defined Sending Size Limit, the message never reaches the store. Microsoft Outlook® will display a dialog box showing that the message exceeds the Sending Size Limit as shown in the following figure.
Note: It is important to remember that the initial client side check is for the Sending Limit only. The Receiving Limit will not be considered (it cannot be overridden) if this Sending Limit is exceeded during submission. Administrators that relied on a larger Receive Limit pulling messages through the system when the evaluation took place in categorization could be surprised when this stops working after applying the fix referenced by Microsoft Knowledge Base article 836738, "E-mail messages that are larger than the "Sending message size" delivery option are delivered in Exchange 2000 Server."
Setting an Organization Wide restriction for Receiving Message Size Limit directly affects the Organization Wide Sending Limit Size. For example, if you set a Global Receive message size limit of 1 megabyte (MB) and leave the Sending Limit not set (unlimited), some administrators may expect users to be able to send messages larger than 1 MB.
Note: The message size limit that is configured in the Sending message size option limits the size of messages that can be sent. The message size limit that is configured in the Receiving message size option limits the size of messages that can be received. These message size limits are applied to the whole organization. These sending and receiving message size limits also affect messages that are sent to and that are received from recipients who are not members of the Exchange organization.
The categorizer applies the globally defined Receive Size Limit restriction to local and remote deliveries. A message destined to a local recipient or external non-organization (Internet) recipient that exceeds the globally defined Receive Size Limit will generate a 5.2.3 delivery status notification.
For the categorizer, any message that exceeds the Max Delivery Content Length and was not overridden on a per-user basis because the recipient did not have a Receive Limit configured, is outside of the configured limits and the 5.2.3 delivery status notification results.
For more information, see Microsoft Knowledge Base article 298572, "An e-mail message that is larger than the sending message size limit or the receiving message size limit is not delivered."
Global Receive Limits are evaluated by the categorizer for local and remote deliveries and cannot be overridden by a larger Sending Limit on any user object.
Use Global Receive Limits with caution.
Global Receive Limits do not work as most people expect. For the categorizer, there are two kinds of global size restrictions:
Max Submission Content Length (SubmissionContLength)
Max Delivery Content Length (DelivContLength)
The latter is applied to all deliveries, including remote deliveries.
As an illustration of this, consider setting a sender mailbox Send Limit to 10 MB, with no Receive Limit set, and then set a Global Receive Limit of 5 MB. Send a message from the sender containing a 6-MB attachment to any recipient.
A common expectation is that the recipient receives the message because the sender had an explicit Send Limit that exceeds the Receive Limit. However, the operation results in a 5.2.3 delivery status notification.
For the categorizer, the message size exceeded the Max Delivery Content Length and was not overridden on a per-user basis with a Receive Limit.
No message will be received that exceeds the globally defined Receive Limit, unless this is overridden on a per-user basis with an explicit Receive Limit.
A user with a per-user Send value that exceeds the globally defined delivery value cannot send a message larger than the globally defined Receive Limit unless the Recipient has a (DelivContLength) that overrides the globally defined (DelivContLength).
The fix referenced by Microsoft Knowledge Base article 836738, "E-mail messages that are larger than the "Sending message size" delivery option are delivered in Exchange 2000 Server," does not help in this case because the (SubmissionContLength) value is not exceeded by the sender.
A per-user configuration for (DelivContLength) overrides globally defined (DelivContLength) settings if they are set.
Increased overhead of messages with binary attachments traveling between routing groups over SMTP needs to be accounted for when implementing restrictions.
For outgoing SMTP messages being sent between routing groups, Exchange Server will render attachments in the Transport Neutral Encapsulation Format (TNEF), which is base64 or quoted printable rather than binary. This will cause an approximate 33 percent size increase in the messages.
This issue can be solved by increasing the global message restriction size by a factor of approximately one third to accommodate the overhead.
In Exchange Server 2003 native mode, Summary Transport Neutral Encapsulation Format (STNEF) is always used for intra-organization e-mail. Therefore, there is no increase in the message size when going through a routing group connector. In mixed mode, the behavior is unchanged, because STNEF is within routing groups only.
As an example, set a Global Sending Limit of 10 MB and Global Receive Limit of Not Set. Also, do not configure explicit per-user size limits.
Put three users in two routing groups. User1 and User2 are in one routing group, and User3 is in a different routing group connected through SMTP. Send a message from User1 to User2 and User3 containing an 8-MB attachment. You may expect that User2 and User3 would receive the message. However, the actual results are that User2, who is within the same routing group, receives the message as expected, but the message to User3 returns a 5.2.3 delivery status notification.
In this example, User1 did not override the Global Sending Limit with an explicit mailbox Sending Size Limit, so the global limit is enforced.
When the 8-MB message crossed the routing group boundary through SMTP and arrived at the destination server, it was approximately 33 percent larger than the original message because of the inter-routing group SMTP increase. The message to User2 exceeded 10 MB on the inter-routing group hop when SubmissionContLength was evaluated by the categorizer on the receiving server. The final message had a content size equal to 11,594,558 (11 MB), and the message exceeded the 10-MB Global Limit, thus returning the 5.2.3 delivery status notification.
The fix referenced by Microsoft Knowledge Base article 836738, "E-mail messages that are larger than the "Sending message size" delivery option are delivered in Exchange 2000 Server," does not help in this case because the Max Submission value is not reached until the message crosses the inter-routing group boundary.
For more information, see the following Microsoft Knowledge Base articles: