Sent emails missing on specific days

Dewa_ 

Is it on your own personal mailbox or shared mailbox?

Are they recent sent emails or 6 months older emails or very older ones since we have a settings to view old emails

Steps

To confirm this, you can go to "FILE"> "Account Settings"> select your mailbox> Change> Exchange cached mode > there if it is "1 month" or "2 months", you can set it to be "3 months" or latter based on your missing sent items email period. 

Also have you to tried search the emails under Advance Search Options on your Outlook Client sometime there are chances that you might have accidentally moved via Inbox Rules or to Online Archive mailbox.

Steps

This step is to check if the mail older than 90 days has been moved to other folder of your Outlook. Kindly note that you should select the "current mailbox" or "all Outlook items" while searching. 

Alternatively, just try to search them on your Webmail as well to see if you can find them there.