Can you block a whole domain like “comcast.net”?

You don't need to use another Mail client like Outlook, you can create a Rule in Apple's Mail client to block a whole domain however remember it will block EVERYTHINGS with that domains name in it. To coin a term, please be careful what you wish for.

You should enable and train the spam/junk mail filter:

Reduce junk mail in Mail on Mac - Apple Support

You can also do the same thing with your email provider.

I find filtering usually gets rid of the majority of spam, with most of the other unwanted emails for me are ones that I accidentally signed up for or someone else signed up for me for work.

It is dangerous to filter a whole domain, because there are a lot of legitimate users sending mail from a Comcast domain.

Sure, again create a Rule. Please read Use rules to manage emails you receive in Mail on Mac - Apple Support

Do the junk emails have 'unsubscribe' at the bottom of the message, if so click on it and unsubscribe. This is only a sensible option if you are sure that you are on a legitimate subscription list.

You can access Rules in the Mail system preferences and you should be able to create a rule to capture any mail from comcast.net and file it in Junk or Bin.

Remember that if you were to block all of comcast.net, you would also be blocking anyone who happens to have Comcast as their ISP and happens to be a friend or business associate.

I set up a rule that all email from Comcast goes to my "Comcast" folder "on my Mac."

If you are already using Outlook, you can do Rules with that as well.