In Outlook desktop app navigate to Home > Junk > Junk E-mail Options > Options > set to No Automatic Filtering. We do recommend having it OFF, as it may move emails to the Junk folder without any reason. If messages from internal senders end up in Junk folder it might be caused by the following local reasons:
Due to the architecture of Exchange servers, the emails sent internally (within your organization) are not filtered for spam.
