When discussing how to select which email addresses should be including in a mailing list the following comment was mentioned to me:
My response was that I believe it's best to do everything you can to protect the reputation of your mail servers and domain name, and that opt-in is more correct even if it's likely to have fewer subscribers. I don't believe it's worth the risk of spam complaints to email everyone on a list of addresses, including those who are not expecting the email, hoping it generates some extra web site visits.