How to add all incoming emails to a list

Hello all, I'm wondering if there's a way to add any incoming emails to an email list. Preferably not using aweber or mailchimp, but I did just sign up for mailchimp so I can use that also if necessary. Point is, I have lots of emails from potential customers coming in every day, yet I don't have a good way to save them all without spending a ton of time, which starts to seem like it isn't worth it. Also my other question is, is this legal? If they haven't agreed to sign up for a newsletter or anything, but have sent me an email in the past, am I aloud to send them offers and stuff?