Anyone tried mailing to purchased lists?

For one of my clients, he says he found someone who is offering him a large list of emails of his target audience. I'm worried about how to go about sending mails to purchased lists without it looking too spammy.

Does anyone have any positive or real life experiences with such email marketing? Does it work and if so how do you go about it ( any tool? ) and which email ID is best to use as an alternative other than his main domain?

It falls in the gray area... Not going to be easy for you...

I tried something similar long ago ... but since I was using MailChimp, they stopped the campaign soon after I started... Not sure how it will work with other providers.