Exchange Server 2003 Message Size Limit

Setting Message Size Limit in Exchange 2003 can be rather tricky and complex. I have been doing research and testing it out for the past few days and here the result of my research and testing.

Here are some useful links to start with.
How to set size limits for messages in Exchange Server
Exchange 2000 Server and Exchange Server 2003 Message Restrictions
Option to strip attachments for messages that generate an NDR
A message that exceeds the configured size limit is sent to a server that is running Exchange Server 2003
Delivery status notifications in Exchange Server and in Small Business Server
SMTP Transport Components

There are four places where you can configure message size limit.

Global SettingThis setting will affect your whole Exchange Organization. The figure below shows that the send/receive message size had been restricted to 131MB for the whole Exchange Organization. This setting will also affect the message size limit of sending and receiving Internet email.




Connector Setting
This setting determines the maximum size of the message sending through a connector. Therefore it is use to restrict outgoing message passing through a connector. The figure below shows that message passing through this connector must be less than 10MB.

SMTP Virtual Server SettingThis setting determines the maximum size of the message passing through the SMTP Virtual Server. Here are some things to take note of.

• All incoming messages will be affected by the setting.
• Outgoing messages sending using MAPI client (e.g. Outlook 2003) will not be affected. I try telnet to the SMTP server and send message large than the configured size, the submission was rejected. However, by using Outlook 2003, the message passes through.
• Setting the size too small might prevent proper Offline Address Book replication. See "You cannot download the offline address book in Microsoft Outlook in Exchange 2000 Server".

User Mailbox SettingThis setting determines the maximum size of the message which users can send/receive through their mailboxes. The figure below shows that the send/receive message size for the mailbox is set to 8MB.


So with all these settings all over the place, how do we know which will take precedence? According to the article "Exchange 2000 Server and Exchange Server 2003 Message Restrictions" , this is how it works.

• User Mailbox sending message size configuration overrides Global sending message size configuration.
• User Mailbox receiving message size configuration takes precedence over Global receiving message size configuration.
• User Mailbox sending message size configuration does not override Simple Mail Transfer Protocol (SMTP) virtual server limits.
• User Mailbox sending message size configuration does not override connector defined limits.
  

Here are some tests and results that I performed.
Test 1
The send/receive message size limit was set to 10MB on the Global setting. No other limit has been set. UserA’s mailbox in Routing Group A and UserB’s mailbox in Routing Group B. UserB tried to send a message that is more than 10MB and receive an Outlook alert dialog box saying the message cannot be send.
If you are using cached mode, you will need to delete away the message otherwise you will keep receiving the alert.


The message was rejected prior to submission which means that the message will not get into the store. If you are using cached mode, your Exchange server and Outlook 2003 must be on Service Pack 2 otherwise, the message will get send out and come back as non-delivery report (NDR).
This simple test shows that the Global setting is working.

Test 2
The send/receive message size limit was set to 10MB on the Global setting. The send/receive message size limit was set 20MB for both UserA and UserB. UserA sent a 14MB message to UserB and the message was delivered to UserB successfully.
Changing the send/receive message size limit on the Global setting to 50MB and the send/receive message size limit for UserB to 10MB. UserA sent a 14MB message to UserB and received an NDR with the following message.
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.
<e2k3servera.contoso.com>
The NDR was generated by UserA’s Exchange server and the message was not routed to UserB’s and this is really good for the network bandwidth. The NDR also does not include the original message which otherwise will be 14MB instead of just a few KB. This is so because the Exchange server with Service Pack 1 or above will automatically strip off the original message if the size is more than 10MB. If you want to customize the size limit, please see "Option to strip attachments for messages that generate an NDR".
This test shows that the precedence between User Mailbox setting and Global setting is working as stated.

Test 3
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.
<e2k3serverB.contoso.com #5.2.3 smtp;450 5.2.3 Msg Size greater than allowed by Remote Host>
The NDR was generated by UserB’s Exchange server and the NDR does not contain the original message.
This test shows that message size limit of the User Mailbox or Global setting does not override the SMTP Virtual Server setting.

Well, there are many more scenarios but I don’t think I am able to test all of them.
Before I end this posting, another thing to take note is the MIME conversion which can cause message send across routing group to increase as much as 30%. So this is also something to consider when implementing message size limit.
The send/receive message size limit was set to 80MB on the Global setting. The send/receive message size limit was set 30MB for both UserA and UserB. The limit message size on UserA’s SMTP Virtual Server was set to 20MB. UserA sent a 23MB message to UserB and it was delivered successfully. UserB tried to send a 23MB message to UserA and received an NDR with the following message.

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