What do to with email list

Hi,
I recently was given a list of quite a bunch of email addresses. Unfortunately I have no idea how the list was built (i.e. if they are opted-in somewhere). So I am wondering what I can to with the list.
Is it even (legally) usable for anything ? Can I just add them to my mailing list, or should I write them a single mail, redirecting them to my site and they can opt-in there so I can work with them ? (obviously I will lose quite a few emails if I re-opt-in them in this way, since not everyone will signup. Is it even legal to send them this kind of email ?)
Does it make sense just to throw them away or try to sell them for cheap and start over building a new list ?

In short: I want to know what I can do with all those email addresses, because it would be quite a shame to just throw them away, since afaik they should be all good and active mail addresses.

Thanks,
Scoville5579

what size is the list and how much did you pay for it? where did you get it from?

they are most likely not worth anything. Nobody gives away a list that makes money.

You could send to it, but I don't think it would be legal as you don't have permission.
also as said above, it's unlikely the list would have been given away if it had any value.
also the value is even less if you don't know the profile of the recipients.

As You said You don't know where those emails came from. Just blast couple of different newsletters and promote your affiliate link and see how much you can make from which affiliate offer. (Note: Send couple of offers of different niches and monitor the opening rate and click through rates of your affiliate links, this gonna be tough job if you want to classified emails by niches)

If you don't know the source of the emails, they are useless.