4 answers
With SFO 2.9 and up (any version of it with the search bar), you can attach an email and (depending on the configuration your admin has setup) its attachments to any Record that shows up in the search results. In the screenshot below I've highlighted an email from a known/active Contact who's not associated with any Opportunities/Cases/Etc. Keeping that email highlighted, I did a search for "test". As you can see, I'm able to associate the email with several unrelated Records.
Thanks,
Ben