Do your best to get your emails into their inbox.
You're going to want to find a list of keywords that Gmail and other free email providers consider spam.
If you're in the make- money online niche, then there are all kinds of landmines you've got to avoid to get into the user's inbox.
Luckily if you're not in this niche, it won't be as big of a problem.
But, you do want to be especially careful of the language of your emails titles, or subject line. A suspicious title will land your email in the spam folder.
There are some email testing tools out there that will give you an idea how spammy, or not, your emails would look to mail servers.