An Email service is required to enable email to be delivered successfully. Any email sent from the your server will come from an unknown source and so will be considered spam by all the major email providers. Email services such as Sendinblue, and SendGrid have free plans that will deasily cover the contact and transactional emails sent by website.
Sendinblue can be configured via Magento admin, Sendgrid requires setting up at the server level.
- Sendinblue Email Service
- SendGrid Email Service