Marketing email SPAM is a complex issue. There are now easy solutions and almost all ISP’s have different parameters:
Gmail for instance likes DKIM based authentication. This may not be a strong criteria for other ISP’s. Gmail it appears also likes the list unsubscribe as part of the email.
Also; recipient engagement is critical for Gmail. At Gmail, engagement is seen as the sum of total user's actions upon the email. Gmail tracks explicit user's actions (marking email as spam, replying to the email, moving it to the Junk folder etc.), and these explicit actions have a direct relation to how Gmail classifies the email for the given use. If your subscriber opens emails from you on a consistent basis, that shows engagement and favours Inbox placement for the current and next email campaigns. But if the subscriber leaves your emails unread, deletes them without opening or moves them to spam, then Gmail will penalize you because your subscribers don’t appear to be engaged with your emails.
Also, simple things like not having too many images or hyperlinks or not using URL shortening services like bit.ly help. Due to their propensity for abuse email messages with shortened URLs have the highest spam rates.
Other best practices include list hygiene and ensuring double opt-in. For example: Hotmail or live.com has a history of inconsistent SPAM filtering and can tend to appear overtly aggressive. Apart from the standard guidelines similar to Gmail, helpful approaches for live.com are outlined below:
Ensure double opt-in
Incentivise recipients to actively engage (such as by replying or via open and click-through)
Request recipients to marking the sending domain (sender address) as trusted
Include an RFC2369-compliant List-Unsubscribe header containing a mailto: address. com only enables this feedback via email, so URIs for other protocols such as http will be ignored.
Ensure a valid reply-to address is part of the email. Default addresses such as “noreply@...” or “do-not-reply@...” tend to increase SPAM capture.