Paris: Emails are used in websites for various purposes; it might be for a simple contact form or to manage subscriptions. One main reason for such mails getting blocked is because it might have been originated from a site hosted by a third party service. So it’s advised to have your own email server or a service provider like SendGrid. This has been built to serve developers while making it easy to send mails over SMTP and HTTP within minutes.
Why to use SendGrid? Read more about How to Integrate SendGrid with WordPress and Drupal?