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.