If you are able to connect to our POP3 server, but not to our SMTP server, it is likely that your ISP is blocking you.
Many ISPs will block you from sending out emails using a different SMTP server than their own on port 25.

Therefore, all customers may use port 26 to send outgoing emails if they need to.

To change this you need to access your account settings in your email software.

Outlook Express:
Tools > Accounts > Properties (on your account) > Advanced.
Change Outgoing port to 26.

Thunderbird:

Tools (Edit under linux) > Account Settings > Outgoing Server (SMTP) > Edit
Change port to 26.