Is my ISP blocking outgoing mail through your server? (Blocking port 25)
ERROR:
The connection to the server has failed. Account mail.yourdomain.com', Server: 'mail.yourdomain.com', Protocol: SMTP, Port: 25, Secure(SSL): No, Socket Error: 10051, Error Number: 0x800CCC0E.

The cause is due to your ISP. Many ISP's block port 25 to prevent spamming. More and more are doing this every day and it may become standard practice in the future. We have seen cases of first level support at ISP's not knowing that they are blocking port 25, so asking them may or may not get you the correct answer. The way around this is to set your outgoing SMTP mail server to whatever syntax your ISP has given you (e.g.. mail.yourisp.com).
We also run an instance of our mail server on port 26, so changing the setting from port 25 to port 26 in your mail reader may fix the problem. Some mail readers do not have an area to designate outgoing SMTP server. If this is the case putting mail.yourdomin.com:26 as the outgoing server will work just as well. See your mail reader documentation for specific info on changing the settings.