SMTP Server Troubleshooting on EC2 Instance

I'm running an EC2 instance with Sendmail as my SMTP server, but I'm facing issues with sending emails. Although the server is running, I'm encountering 'connection refused' errors. I need an expert to help diagnose and resolve the issue, as well as test the server's functionality.

Key Requirements:
- Diagnose and resolve 'connection refused' errors
- Test SMTP server functionality
- Ensure successful email dispatch

Ideal Skills:
- Extensive experience with EC2 instances
- Proficiency in Sendmail configuration and troubleshooting
- Familiarity with email server testing