Q:

Magento 2 : Transactional mail going to spam (only Gmail)

I have configured SMTP Magetop - Marketplace Extensions using Sendgrid(Third party service) in M2 site(ver. 2.1.8).
When I trying a test for Gmail, mail is going to spam folder. But when I trying using Zoho, Yahoo and office365 the mail is going to inbox.
I did System > Configuration > Advanced > System > Mail Sending Settings > Set Return-Path to YES
As well as I am tried with Gmail SMTP.

A:

No matter what you use for sending emails (third party service via SMTP or server PHP Mailer) this not a Magento related issue, but a mail configuration one.
Even if you use Sendgrid, you might have to configure your DNS settings to authenticate the server where Magento is hosted. Best way to find out the exact reasons why your emails end up in spam is to run a test, and the most useful tool I used for this is https://www.mail-tester.com/

