Why does link magically appear in emails?

I’m getting ticked…

I have been noticing in some emails I’m receiving, and even emails I send out when I include myself in the bc: field that links magically appear out of thin air within my emails. A word or phrase will have a double underline, and will be hyperlinked. When you click on the hyperlink, it goes to a freaking ad.

Does anyone know what the heck’s going on with this? Is this a new way to spread adware, viruses, etc., and if so, how in the world do I eliminate this from my computer?

Thanks for your help!