Course the big question is, how old are these emails? You said "past" customers. If you've never emailed them before and/or it's been a long time since they heard from you, they could be quite unresponsive. It doesn't take long for people to forget you, or that they did business with you, and many of them might click the spam button.
Plus if you have never emailed them, or rarely, many of the emails might not even make it into their inboxs.
I've never imported, so I am no help there. Though I think most places are going to be like Rob hinted at, they will probably want to know how the email addresses were obtained. But what do I know
If you do decide to mail them, at least the first couple emails should be low key, no selling, no promotion, just about building or rebuilding the relationship. If the first email is selling they are probably going to unsubscribe. Well you probably already know that.
That happened to me yesterday. Received an email from a marketer who I had not received any emails from for a long time. I kind of remember getting on their list, but it's like first time I've heard from you in a very long time and you want me to buy through your affiliate link? geez. Gotta at least buy me dinner first.
Needless to say, ubsubscribe.
Actually I think that email was in the spam/junk folder, most likely because they don't email enough.