Managing security settings and protections in Outlook on the web for business
Please note that this forum is not Microsoft support, this is a user-to-user support forum. As I am a normal forum user like you, I can only provide information based on your issue description.
Hi @Robert
Since you mention this is a business/Microsoft 365 organizational account, the issue could be related to an email security policy, mail flow rule, spam filtering, quarantine action of your company.
These settings are configured at organization level, so if a message is being blocked at this level, adding the sender or domain to your personal Safe Senders list may not override organization-wide mail filtering policies.
If you are a normal user in your company, please contact your IT administrator and ask them to perform a message trace for the missing emails.
I look forward for your response.