Exchange - messages are not delivered to distribution groups

In multiple server environments, messages sent using distribution groups may not be delivered to the recipients. When we investigate the message route in the Exchange built-in tool “Message tracking center” we will get the following three lines:

SMTP: Message Submitted to Advanced Queuing
SMTP: Started Message Submission to Advanced Queue
SMTP: Message Submitted to Catagorizer

As we can see, the message was stuck in the Categorizer (component of the Exchange Server which decides how and where the message must be delivered); it has not reached its recipients and we have not received a Non Delivery Report (NDR). In most cases the reason of this is a lack of connection to the so-called Expansion Server i.e. the server that stores the list of objects included in a given distribution group ). Before we get to the details let’s enumerate the distribution group types in Active Directory:

• Domain local/Global – list of member objects is stored on Expansion Server responsible for certain group (selected in group configuration - rys. 1.)

 
Fig. 1. Expansion Server configuration.

• Universal - list of member objects is stored in Global Catalog (a copy is stored on every domain controller with Global Catalog Server role)

When a message gets to the Categorizer its recipients are resolved. If the recipients list includes distribution group, Exchange server will try to to communicate with Expansion Server responsible for this group (resolving the list of objects included in this group). If for some reason the connection to Expansion Server fails, the distribution group is removed and message is aborted (if it doesn’t include any other recipients) without reporting any errors or NDR. The two main reasons for connection problems are:

1. Wrong configuration – global distribution groups require an Expansion Server in the domain the group was created. If this condition is not met, the distribution group will be visible in the Global Catalog but the messages will not reach its recipients because the server responsible for sending the message will not be able to retrieve information on which server stores object data.
2. Temporary lack of connection – sometimes, especially in complex networks, temporary lack of connection may occur (connection latency, DNS server errors etc.) which result in removing distribution lists from the recipients list if they cannot be resolved.

In both cases the best solution is to use universal distribution groups as the data on objects that belong to the distribution lists will be stored in the Gobal Catalog, which speeds up and helps resolve distribution groups. Naturally, it will increase the network activity when the data is replicated between the servers in the Global Catalog but this will rather have a marginal impact.