The main problem with "opt-in" mailing lists is that they are very rarely opt-in. It's quite possible for users to be signed up at an "opt-in" mailing list without wanting to be. To prevent this, MailMask.net recommends all opt-in entries are verified. For example, if user@domain.com signed up for an opt-in list, the server should send an email to user@domain.com asking them to verify that they would like to be added to the opt-in list.
Following this practise will prevent others from typing in false email addresses to be added to the mailing list, and will ensure that only people who really want to receive emails will be added to the list. Be sure to read our list of best practices section for other ways to be an ethical email marketer!