The simple answer is to not send out spam. Each email should be personalized and have a purpose.
You should always work off an opt-in list that you build yourself or a trusted agency builds for you. Purchasing email lists off the internet is a quick way to be flagged as spam because you don't have any connection to the recipient.
Having trouble getting people to opt-in? Offer them something useful for joining such as an eBook or free consultation.
Another tip for avoiding the spam folder is to personalize your emails with the recipients first name and write emails like you are communication with a friend rather than trying to make a hard sell.