How do I get rid of popup ads?

I'm a little confused, since recipients have only limited control over whatever an email message contains.

Open the Mail app's Preferences... menu > Viewing. De-selecting "Load remote content in messages" will limit the amount of potentially unwanted content. You can always click the "Load remote content" option at the top of those messages, if you want to view it.

If you consider the message junk, selecting Message > Move to Junk will dispatch it.