My SMTP server appears to be blacklisted and many emails are rejected as spam

The SMTP server provided by my hosting company ( appears to cause problems – many of my emails sent from this IP address get into the recipient’s spam folder or are rejected by the recipient’s SMTP server.

What are my options for fixing this or using some sort of third-party SMTP server? Except using G Suite (5 USD per month) and except changing the hosting provider.