Check these articles for more insight - https://techcommunity.microsoft.com/t5/exchange-team-blog/email-stuck-in-exchange-on-premises-transport-queues/ba-p/3049447
https://learn.microsoft.com/en-us/exchange/troubleshoot/mailflow/email-stuck-queues
https://practical365.com/exchange-2016-outbound-mail-flow/
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