Mail delivery is slow after you configure delivery restrictions that are based on a distribution list

If the following conditions exist, inbound and outbound mail delivery may be slower than you expect:

• You configure delivery restrictions on a Simple Mail Transfer Protocol (SMTP) connector.
• By default, your restrictions are set so that messages from everyone are rejected.
• You add distribution lists to the Accept messages from list.

This problem occurs because Exchange Server must expand the distribution lists to determine whether each user account is permitted to send or to receive the e-mail message. Also, the results of this distribution list expansion are not cached by Exchange Server.

To work around this problem, use one of the following methods.

Method 1: Limit the use of this delivery restriction

Try to limit the number of servers that use this kind of delivery restriction. If you can, enable this kind of delivery restriction only on source bridgehead servers in your organization. If you can limit this to only certain bridgehead servers, try to dedicate a global catalog (GC) server to each of these bridgehead servers.

Method 2: Apply user-based delivery restrictions

Add the specific user accounts to the Accept messages from list instead of using distribution lists. This increases performance because Exhange Server caches the results of delivery restrictions that are based on individual user accounts. Exchange Server does this even though it does not cache the results of the expanded distribution list.

This method works when you want to add up to 1,000 user accounts in a Windows 2000 Server domain controller (DC) environment, or up to 1,500 users in a Windows Server 2003 or Windows Server 2008 DC environment. Therefore, the maximum number of users in each environment is 1,000 or 1,500, respectively. To accept messages from more than the maximum number of users, you must create additional SMTP connectors.

When you create additional SMTP connectors, first make sure that each connector is identically configured. Then, add some amount of user accounts that is less than the maximum number. Add the accounts to the Accept messages from list for each connector. Important Make sure that each SMTP connector is assigned the same cost. Otherwise, Exchange Server queries only the first SMTP connector, and returns an NDR if the user account is not listed in the Accept messages from list for that connector.

For more information about another solution that is available if you are running Microsoft Exchange Server 2003 Service Pack 2 or a later verion of Exchange Server 2003, click the following article number to view the article in the Microsoft Knowledge Base:

Microsoft has confirmed that this is a problem in the Microsoft products that are listed at the beginning of this article.

For more information about how to enable a distribution list-based restriction to a connector, click the following article number to view the article in the Microsoft Knowledge Base: