I'm no expert but here are my suggestions.

1. Long story but try to go to one of the Microsoft chat rooms for a solution. In a recent incident with Microsoft - and Outlook - I was told I needed more memory since I opted out of using its cloud service. The sales pitch has come again that I've "run out of memory." My suggestion is buy memory for a month, see if the yahoo email is there, and then drop Outlook as a backup email.

2. My long term suggestion would be to get out of Outlook as a backup - and Yahoo also - and go with Proton Mail for your session email. If needed get another free email, Gmail from Google for instance.

Good luck!