Hi @Jerry Su ,
e.g. one external client send an email to my company with 5 recipients and cc to 10 recipients.
show to count the message in this case, 1 message? 5 messages ? 15 messages ?
Per my understanding, this should be counted as 1 message.
According to the diagram below and the explanation cited from this document, the inbound message enters the transport pipeline through the default Receive connector named "Default Frontend <Mailbox server name>" in the Front End Transport service, then the Transport service listens for messages on the default Receive connector named "Default <Mailbox server name>". The categorizer which is responsible for recipient resolution and routing resolution works after the message is processed by the receive connectors. Therefore, the message in your example is counted as one and is not related to the number of the recipients.
Hope this can be helpful.
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