Anything where the initial step of someone trying to get you into anything financial is to send you an e-mail.
There are valid situations in which e-mails may be used to introduce you to a financial product or offer, such as if you have signed up for an electronic newsletter that includes such information. But in that particular case, the e-mail isn't the first step; rather, whatever caused you to sign up for the newsletter was.
Even in a valid, legitimate scenario, you should obviously still perform due diligence and research the offer before committing any of your money. But the odds that someone is contacting you out of the blue via e-mail with a legitimate financial offer are tiny.
The odds that a lawyer, a banker or someone similar in a remote country would initially contact you via e-mail are yet smaller; I'd call those odds infinitesimal. Non-zero, but unlikely enough that it is probably more likely that you would win the grand prize in the state lottery four times in a row.
Keep in mind that responding in any way to spam e-mails will simply confirm to the sender that your e-mail address is valid and is being read. That is likely to cause you to receive more spam, not less, no matter the content of your response. Hence, it is better to flag the e-mail as spam or junk if your e-mail provider offers that feature, or just delete it if they don't.
The same general principles as above also apply to social media messaging and similar venues, but the exact details are highly likely to differ somewhat.