Hi @Mohamed Aly ,
Could run the get-message tracking command to get the detail of the message?
Please confirm that you have more than 10% free space on your exchange server. Mail would also stuck in the queue if the server do not have enough space.
You could try to restart the transport service to check if the emails is released.
I also find a similar case for you reference.
emails pending delivery (microsoft.com)
In this case, Op re-created the receive connector and then solved the issue. You could also try this way to check if the issue get resolved.
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