Here's the "secret":
Sorry to disappoint… but if you use an email marketing tool / ESP to send emails, there will always be emails that land in the promotion tab. It is the way it is. You can search for this on Google.
An email lands in the promotion tab because Gmail thinks the email is promotional. It will be moved to the primary inbox of the recipient if he/she often engages with emails from the same sender.
The emails can be moved again to promotions tab if the recipient stops engaging with the emails, or because of something else. Like SEO and social media, mailbox providers can change their inboxing algorithms any time.
See this comment by Anne Mitchell, an email deliverability expert who coined the term "email deliverability":
Can you do something to REDUCE (not prevent) this?
Yes, follow the guidelines listed in the previous “reduce emails going to spam” section.
Plus:
- Ask them to also check their promotions tab if your email is not in the primary tab
- Ask them to drag and drop your email to the primary tab (if it’s not already there) OR
- Ask them to add your email address to their address book
There really is no “secret” 😜