can my devices be infected with spyware from a suspicious email?

Unlikely. All of Apple's devices sandbox the web browsers so they do not access to the rest of the operating system.

There are situations where a web page (often times an Ad on the web page) gets some Javascript into the web browsers cache and keeps throwing up pop-ups that try to scare users. Generally the solution to these is to clear the browser's cache.

Generally yes, clearing the browser cache stops the pop-ups until you browse to a site that introduces that or similar Javascript into the cache again.

For an iPhone/iPad, Settings -> Safari -> Clear History and Website Data

For a Mac, quit Safari. Hold the "Shift" key while launching Safari, and that will clear the Safari caches.

If you use some other browser (Firefox, Chrome, etc....) then you may have to either check their preferences, or Google how to clear their caches.