Here's a guide to assist our customers having concerns around email placement in Gmail's folders such as 'Promotions' as well as understanding deliverability when using Customer.io platform.
A common report from senders is that many of their emails sent from Customer.io (or any other messaging platform, for that matter) tend to land in the recipient's 'Promotions' folder in Gmail, even when the content or subject line is not promotional. The issue arises from a desire to manage email classification on the recipient's end as well as monitoring where emails end up without manually querying every individual recipient.
Customer.io does not have control on the particular Gmail folder (Primary, Social, Promotions) where the emails land. This is primarily a decision made by Gmail's algorithms, which are not disclosed to the public. However, there are strategies to improve the chances of landing in the 'Primary' tab. Encourage your recipient to manually drag the email from 'Promotions' to 'Primary' tab or ask them to add the sender's address to their address book.
A high level of engagement with the email content over time can potentially improve the sender domain's reputation hence improving email deliverability. For continuous monitoring of Gmail-reported spam rate and reputation, we recommend setting up Google Postmaster Tools.