Drain the mail queues on the server.

I want to put the Exchange 2016 server into maintenance mode before rebooting it. The first command is to  Drain the mail queues on the server.  

Set-ServerComponentState EX16-1 -Component HubTransport -State Draining -Requester Maintenance

Question:  What does it mean by draining the mail queues.  What would this command do on server? I want to understand it practically . Can some one explain please.
Muhammad AsifSenior Solutions ArchitectAsked:
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Adam BrownSr Solutions ArchitectCommented:
When you set the queues to drain, they'll stop accepting mail and finish processing whatever mail is in the queue. This keeps you from losing or delaying mail during maintenance. In this case, you are tagging one of the servers as being in maintenance mode, which shuts down the transport service on the server so it does not accept SMTP or x.500 messages for delivery on that server. This will cause any servers that are attempting to send to that server to do one of these options:
1. deliver to whichever server is on the next priority MX record
2. hold on to the message and await restored service on the server
3. route messages through a different server

Usually it'll be number 2.
Todd NelsonSystems EngineerCommented:
From a high level, "draining" essentially refers to pushing the messages from that server's queue to one of the other active servers in the DAG so the outbound messages will get delivered normally if the maintenance window takes longer than expected.
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