Hi @Anonymous ,
Could you find the missing emails in the archive mailbox? When you enable the archive policy for 1 year, emails in send items that are older than 1 year will be automatically moved to the archive mailbox and they will no longer appear in send items.
For in-place archiving you can refer to the documentation:
https://learn.microsoft.com/en-us/exchange/policy-and-compliance/in-place-archiving/in-place-archiving?view=exchserver-2019
I apologize if I misunderstood you. You could describe your issue specifically and then I will try to help you solve it.
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