Sending, receiving, and organizing email in Outlook.com
Sorry to hear about this situation. The Arlo app is a third-party service, so being signed out of it was not an action performed by Microsoft or Outlook. You will need to work with Arlo regarding the sign-out issue.
You can add the exact sender addresses or domains provided by Arlo yourself by going to Outlook.com > Settings > Mail > Junk email > Safe senders and domains. Microsoft provides the instructions here: Safe Senders in Outlook.com.
After adding them, ask Arlo to send another message. If it still does not appear in either your Inbox or Junk Email folder, Arlo must check its delivery and rejection logs. If Outlook.com is rejecting its messages, the administrator responsible for Arlo’s email system will need to contact Microsoft Sender Support; this cannot be corrected from the recipient’s account.