Is Reagan.com mail incompatible with MacMail?

Just drag it out of Junk and / or mark it as "not junk". The Mail app will learn.... eventually.

If you are so motivated, you can create a Mail Rule that will send it to your Inbox: Use rules to manage emails you receive in Mail on Mac - Apple Support.

Something like this might work, but you will need to adapt it to the important email messages you receive:

If Reagan.com is an email standards compliant email server, then it is compatible.

Compatibility has very little to do with the amount of spam you get, though more legitimate email servers will tend to not have email messages marked as spam as it passes through the various email exchangers .