Hello colleague.
I don't recommend it at all if you are going to use it for mailing.
It is true that it depends on how you use it, it is legal, but be very careful because you can get into a mess if your messages are very commercial. But in addition to that, he barely converts that method, the client is very cold and also by text that they still pay less attention than by phone.
What I recommend is if you want to download the emails, make a personalized audience on Facebook by importing those emails and advertise what you offer to that personalized audience. You'll spend something more but you'll convert more, you'll see.
Regards, partner!