Email programs (like Gmail, Yahoo, Exchange) rely on many factors to determine if an incoming email is spam.

Here are the key factors

1. Is the sender in the contact address book? (it is not required but being in recipients contact book will ensure that your emails are never going to be treated as spam)
2. Did the recipient flag a previous email from this sender as spam?
3. How many recipients have flagged previous emails from sender as spam? (it is more than likely that over 10% of your recipients have gmail addesses. So Gmail can review how other recipients have treated your email)
4. Does content of the email have words that commonly appear in Spam emails?

Improve deliverability and avoid emails going to Spam

Here’s the set of suggestions to help you avoid spam

1. Send great content
2. Only send to those that have opted in to receive your emails. Those that expect to receive your emails are less likely to click the ‘Spam’ button the moment they see your email.
3. Authenticate your emails with SPF and DKIM