One of my users noticed that they are missing messages from their sent items. We went through his sent messages and have determined that the missing emails typically have attachments that are 5 MB or larger. We use O365 Exchange Online and the Outlook desktop app for sending and receiving emails. My guess is that there is some type of retention policy. However, I am unable to locate a policy, and I have not made any configuration changes to the default settings. Any suggestions?
If you are the admin and have the permission of E-discovery manager you can run an discovery search on the recipient mailbox and find the missing emails provided there was a retention policy or litigation hold or in-place hold on the mailbox .
Alternatively , you can also search your retention policies and see if there is no policy resulting in that behaviour . You can go to Exchange admin centre then to compliance management and then retention policies