Office 365 deleting emails by itself

Hello Team,

we use outlook client to connect to office365, they are not configure as pop3. a few users continuously complain that emails gets deleted by itself.

during the troubleshooting i disable this features

set-CASMailbox -Identity user@domain.com -ActiveSyncEnabled $false

set-CASMailbox -Identity user@domain.com -MapiEnabled $false

when both features are off emails do not get deleted but when they are enable then computer behave weird, AV shows no sign of viruses. user do not have any rules, and emails gets deleted whether they use outlook or cloud web portal.

please advise if anyone has experience similar behavior.

thank you,

Geovany