I do not how to make EM Client work for an email account with SMTP with security policy STARTTLS on port 25).….

As a result of it, at the end of the email account setup process, I manage to receive emails but not to send emails.

I have also tried different configurations of Security policy to overcome this problem, but it didn^t work. Has someone found a solution?

Many thanks!

The security policies are specific to the port. The combinations for SMTP are:

Port 587 = Force usage of SSL/TLS
Port 25 = Use SSL/TLS if available
Port 465 = Use SSL/TLS on special port (legacy)

Port 587 is the best for security, and port 25 is the worst. 

If none of those combinations work, disable your antivirus application and then try again.