Hi,
This issue may be related to many aspects from both server side or client side.
- Does the issue occurs when you try to send an email to particular recipient?
- Please try exiting Outlook desktop client and see if the issue can be reproduced on the web mail. So we can determine whether it is a server issue or a client issue.
- If it perform well on the web mail, please try to empty your Autocomplete list in your Outlook client and test again. (Go to File > Options > Mail > Send messages > Empty Autocomplete List)

Any update, please feel free to post back.
Regards,
Aidan Wang
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