Using classic Outlook for Windows in business environments
Hi, Darren.
I'm Jen, I'm also an Outlook user just like you. I'm not working for Microsoft but I'd be happy to help you figure this out.
I understand you're unable to receive emails from a specific sender. Aside from adding the email address to the safe sender's list, I suggest following the methods listed in the "I didn't receive an email someone sent to my Outlook.com account" section in the link below to help you further identify what may be causing the issue.
https://support.microsoft.com/en-us/office/fix-...
I hope this helps. Feel free to share how it went.
Kind regards.