[answerman]

Most times this has to do with port 25 blocking, which someone else mentioned. I don't remember which provider you are using as your SMTP server, but most reputable mail services provide an alternate port (the "standard" alternate port for most providers is 587, which is what Gmail uses, while Yahoo uses 465).

To specify the port in your account settings, append it to the SMTP server with a colon like so:

Outgoing mail server: smtp.my.provider.com:587

Give that a shot and see what happens.