I’d define a clean list as one where:
- Everyone on the list has explicitly opted in to hear from you
- You’ve emailed it in the past 3 or 4 months
If it’s been 9 months since you’ve emailed your list, I would send the email and as the first sentence include verbiage:
“You’re receiving this email because you signed up to hear from us at www.example.com. If you’ve changed your mind, click the unsubscribe link at the bottom of this email and you won’t hear from us again.”
If you haven’t emailed the list in more than a year, use a service like NeverBounce or BriteVerify to clean the list (remove any emails they deem sketchy).
If you have purchased a list, don’t use a bulk email service like MailChimp or Drip. Purchased lists are bad news (read: lots of spam complaints) since the people receiving the emails don’t want to hear from you.