How does Gmail categorise the email messages in my inbox as primary, social, etc.? What should I do to make my mails send from my application be in the primary box always?
Gmail won't tell you exactly how it filters messages into each of the tabs/categories, but the overview is this:
Primary - Email from your contacts, friends and family, and those that didn't filter into other tabs.
Promotions - Specials, deals, coupons and similar from stores, shopping carts and the like.
Social - Social networks like Twitter, Facebook, and sharing sites like YouTube, Soundcloud, dating websites and gaming portals, etc.
Updates - Usually bills, receipts and confirmation emails.
Forums - Notification emails from forums, discussion groups, mailing lists, etc.
You, as the sender, cannot really dictate which filter tab your email appears.
Not unless you tell your recipients to change it on their end. It has to be done per person getting your email. They need to filter your email explicitly into the Primary tab.
They can do things such as:
- Drag and drop the emails from one tab over to another
- Right-click on the message and Move to tab → Primary