Most likely your connection to the SMTP server is blocked by your ISP or your ISP's mail server is down.
If you're using SMTP services provided by Southo.net you will need to set your email client to authenticate (provide username and password) if it's not already.
Another reason may be that your ISP is blocking port 25. In this case, set your mail client to use port 587 for SMTP.
Consult your email client documentation for details on where to change these settings
