Exchange 2019 custom address list the number of members limit

In Microsoft Exchange Server 2019, the maximum number of members in a custom address list (also known as a custom global address list or GAL) is determined by the maximum size limit of an Active Directory distribution group.

By default, an Active Directory distribution group can contain up to 10,000 members. This limit can be increased up to a maximum of 100,000 members by modifying the Active Directory configuration.

It's important to note that the actual number of members you can have in a custom address list may also be influenced by other factors such as server performance and available system resources. Additionally, managing very large address lists can have an impact on the overall performance of your Exchange environment.

If you need to have a custom address list with a larger number of members, you may need to consider alternative approaches such as dividing the members into smaller groups or using dynamic distribution groups based on specific criteria. These approaches can help you achieve your desired organization of recipients without hitting the maximum limit.

Please keep in mind that the information provided is based on the capabilities and limitations of Exchange Server 2019 as of my knowledge cutoff in September 2021. It's always recommended to consult the official Microsoft documentation or the latest version of Exchange Server for the most up-to-date and accurate information.

Hi @CHUN HSIEN KUO ,

I am upgrading Exchange 2019 from CU12 to CU13 to see if it can help troubleshoot the issue.

Sure, you can go ahead installing CU13 and see if the issue can be resolved. In case the problem persist, could you please confirm things below?

1. As aforementioned, is this a recently occurred issue? Any changes were made into your Exchange organization right prior to this issue?
2. How about changing the language of the mailbox in OWA to a different one like Chinese (Simplified) and see if the error remains?

In addition, based on what I've learned, similar issue may occur when a Windows server 2022 is introduced into the environment as a GC, if this applies to your situation as well, I'd recommend switching GC to Windows Server 2019 and check the result.

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Hi @CHUN HSIEN KUO ,

I created an address list with over three hundred members, and it is displayed correctly in both OWA and Outlook address book. However, when I created an address list for the entire company with approximately seven hundred members, OWA displayed "unable to fulfill your request. Please try again later." Outlook displayed "insufficient system resources, please contact your administrator."

By " address list", you are referring to the contact list created in OWA or Outlook, right? If this is true, is appears to me that what you described is actually the expected behavior. It's probably related to the fact that, for mailboxes hosted in Exchange 2019, the maximum number of recipients in a message is only 500 by default, see this link:

And when you use the contact list in Outlook or OWA to send mails, every member of the list would be counted as a recipient.

With the above being said, agree with @Anonymous that it's suggested to create the distribution list (distribution group) from the Exchange admin center instead (supposing you are the Exchange administrator) because a distribution group counts as a single recipient.

More detailed instructions about how to create distribution group in EAC, you may refer to: Manage distribution groups.

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