SMTP Sever Error-Cannot Send email

All of a sudden I am unable to send emails. I can receive emails.

When I run diagnostics, SMTP returns fail. Fix is able to find a fix and fix the problem When I save and close and run again diagnostics SMTP fails again.

I believe the SMTP server error as captured in the log file is
SMTP server didn’t like our greetings and replied with: Unexpected end of stream.

As a test, can you completely disable any anti-virus, firewall, proxy or VPN, then try again.
Sometimes even restarting your router may resolve this.

Thanks, Gary
I tried disabling ant-virus, firewall applications but still had no luck in making the SMTP connection work.
I powered off the router and restarted it. That also did not help. I tried an old desktop I had not used for about six months that had em client installed. Even in that desktop could not send emails. Same SMTP error message.