Spam (in the discussed meaning) is Unsolicited Bulk Email. If it's Unsolicited, Bulk and Email, then it's apam.
Get permission (before sending the first email) and it's not spam.
Confirm that any address given to you is given to you by the legitime address holder, that is -
first mail should not contain anything other then a notice of how you got the adress (IP+time+timezon), and how to confirm this was a legitimate opt-in (normally reply to first mail containg a random token).
If not this foirst mail is replied to - don't add the address to the list, and don't send any more mails.
So: Get persission, from the address holder, and it's not spam.
Getting spam marked [spam], [adv] or whatever doesn't help. The cost (receiving of it) is already taken to the bigger part by then, even if filtered and so never-seen.