Wordpress Email Template

Currently, Our Site is using Post SMTP Gmail API. But all the emails were going to the Junk or Spam Folder. Need someone to fix the E-mail Issue to deliver the E-mail on time and not land into the user Junk or Spam Folder. If you have any other options for E-mail Services, open for suggestions. Also, Need a Feature where we can customize the E-mail Template Completely.