Efforts to Plant Malware through my Apple Mail

Avast has not rebuffed any virus attempt from the email. It is actually useless and is just giving you a false message so you think they are doing something to protect your security and there is actually nothing that they have done. Trying to prove their relevance is how they maintain users and subscriptions.

As for the SPAM email you are receiving, the sender of these spams just spoof the sending address so blocking one will not prevent the next one from coming in. Anyone that has your email address is going to be allowed to send you an email, but you can use email filtering rules for words such as "Unicorn Bulletin" that will automatically move those emails to Junk or Delete without relying on the false sending address.

Use rules to manage emails you receive in Mail on Mac - Apple Support

Get rid of Avast. It does nothing useful and takes your money. It's giving you false positives. Unless you click on links and download things from those emails, they are not dangerous.

Your Mac absolutely has built-in malware protection.

Effective defenses against malware and ot… - Apple Community