Hi Bod,
Based on current situation, please reset the user’s password as soon as possible at first.
Secondly, check whether or not grant another user “full access” permission for this mailbox:
1.Connect to Exchange Online via powershell: http://help.outlook.com/en-us/140/cc546278.aspx
2.Run command below to check permissions on this mailbox.
Get-MailboxPermission -Identity <Mailbox ID 1> | Format-List
3. Remove “FullAccess” permission from the mailbox.
Remove-MailboxPermission -Identity 'UserBeingGrantedPermission' -User 'UserWhoseMailboxIsBeingConfigured' -AccessRights 'FullAccess'
-InheritanceType 'All'
Thirdly, check your antivirus software’s quarantines or disable antivirus software to monitor mailbox.
Thanks,
Jacky Ding