M365: Whitelisting Stuff Against My Better Judgment

 Well, there are a few options on M365 when looking for where mail flow is hanging up.  I've never been really sure why you have to add it in multiple places.  Probably has something to do with the service it hits first in the mail flow.

• Sometimes it helps to add a mail flow rule "admin.exchange.micros>Mail flow>Rules"
•  Add a new rule but just remember to enable it.  
• Another option is to take a look at the SPAM filter "Security.microsoft.com>Email & collaboration>Policies & rules>Anti-Spam policies>Anti-spam inbound policy (Default)"
• Policies & rules > Threat Policies >Preset security policies.  Select the policy you want and then walk through the wizard until you get to the Impersonation protection section.  Then you will eventually come across the trusted senders and domains feature.