As email service providers go, Gmail can be particularly tricky about labeling messages as spam.
Gmail judges email principally on two criteria: sender reputation and recipient engagement. Assuming sender reputation is not the issue (we monitor blacklists daily and would let you know if you had problems), Gmail is likely filtering your emails due to a lack of previous visible engagement with the email by the recipient (opening, replying, forwarding, marking with a star, etc.).
There are two ways recipients can prevent email from being filtered by Gmail:
- Add the "from" address of your emails to their contacts list
- Click the star icon next to one of your emails
Both indicate to Gmail that emails from you are important (engagement).
Additionally, Gmail may filter messages into tabbed inboxes, such as "Updates." To change this behavior and get those messages back into the "Primary" inbox, users just have to click and drag one of those messages onto the "Primary" tab.