While engagement is key at top-tier mailbox providers, Gmail is most notable due to its filtering based not only on a user's engagement with your sending domain, but also how users manage and interact with their own mailboxes as a whole.
Not only may a user be opening and/or clicking the emails they are receiving from you, they may also be considered a "heavy user" in regards to their own mailbox interactions every day. This combination leads to better email placement overall for the user.
On the other hand, someone who engages with your mail, but does not interact heavily with their own mailbox regularly, may experience junk folder placement.
In short, users who interact with mail from your particular sending domain and frequently interact with their own mailbox in general, are more likely to receive your mailings in the Inbox as well as to the proper "tab" or category.
Gmail's secret sauce
While Gmail does not provide us with the recipe for their "secret sauce" (algorithm & filtering components), we do know that there are significant factors outside of engagement that also play a role, such as history of send volume and cadence, complaints and user actions (e.g. moving messages from the Junk folder to the inbox or marking email as "this is not spam").
Gmail also tracks how mail delivered to the spam folder is managed by their customers. If mail is left in the Junk folder with no interaction, this is seen as a negative or lack of engagement and factors into reputation.
At some point early on in the warm up process (initial days of mailing), mail is rejected for all new IP addresses. As the new IP address are warmed up, mail will oscillate between the Inbox and Junk folder so that Gmail may observe how customers are interacting with each specific delivery type of mail.
Gmail is looking at engagement in two separate lights; how a user engages with a specific sending domain, as well as how the user interacts with their mailbox in general.
For Gmail, these two types of engagement go hand in hand as filtering determines Inbox vs. Spam folder placement.
Bulk sender guidelines
Gmail recommends the following in regards to proper email set up:
- Use valid rDNS for IP Addresses sending mail, pointing to a valid domain.
- Use consistent IP addresses and From domains for both Transactional and Promotional mail.
- Publish an SPF record.
- Sign all messages with DKIM.
- Decide on a DMARC policy that works for your programs and publish.
Remember that authentication plays a vital role in Gmail's filtering systems.
Incoming mail categorization
All incoming messages at Gmail are directed to either the Inbox or the Junk folder. Once this has been determined, further classification within the Inbox occurs and mail is placed into the following categories or "tabs":
These "tabs" are enabled by default, but users possess individual control over how their mailbox is ultimately set up.
User actions also help determine future placement, e.g., users moving mail from one tab to another or marking mail as "this is not spam" and moving from the Junk folder to the Inbox.
Based on specific user actions, such as moving mail to a folder or adding a label, Gmail will prompt users to select specific placement for either an individual mailing or all future mailings, helping to create a more customized user experience.
When sending promotional mail, it is common for this mail to be directed to the Promotions tab. This should not be considered a negative. If the user expects all promotional mail to land in the Promotions tab and it ends up elsewhere, the mail may be missed or overlooked.
The categories or "tabs" set up by Gmail have been proven as a positive way for users to keep their mail organized. The ultimate goal should be the Inbox, with mail landing in the proper tab according to category, not just the Primary tab.
At the end of the day, your goal should be to stay out of the Junk folder, while finding your way to the proper tab based on category. With mail being directed to the "tab" users are expecting, engagement is much more likely and will lead to a positive reputation.
Gmail also offers transparency into sending metrics, IP and domain reputation through their Postmaster Tools Site. Once you add and verify your own authenticated domain, Gmail requires proof of ownership through DNS and then allows access to the Postmaster Tools Site.