https://www.whatcounts.com/2012/03/gmail-spam-warnings-and-their-meanings-for-email-marketers/
See above are articles on how to improve email deliverability to Gmail, Yahoo, Hotmail... email servers.
Emails sent to Gmail can be added to a whitelist to confirm thesender's email is not spam, read the jotform.com guide above on how to do this. Unfortunately, you have to request your customers do the white-listing but if you understand what to do, you can explain to your customers
Emails sent to Gmail can be added to a whitelist to confirm the