Hello @St.James.New.London ,
Unfortunately, t his has been a regular occurrence for ESPs industry-wide with Gmail since the introduction of their promotions folders. While our developers are constantly working with email clients like Outlook, Yahoo, Google, etc. to ensure deliveries, this is something that gmail specifically does, that we've little control over.
The only workaround at the moment is to encourage your contacts to check their promotions folder and either flag your emails as okay in their regular inbox (by moving the email to the inbox tab), or encourage them to remove their promotions folder so these email types will default to the regular inbox.
For more information on the Promotions tab in Gmail, and how to get your contacts to mitigate it, I'd suggest checking this article.
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