SMTP stands for Simple Mail Transfer protocol.
You send all of your outbound mail via an SMTP server, and this is normally the server operated by your Internet Service provider, or the company that hosts your domain, if you have one.
In your case, if the email address concerned is the one which you've used to register with us, these are the settings you should enter in the mail account details for the SMTP server:
SMTP Server Address: smtp.mail.yahoo.co.uk
SMTP Port: 465
From Email Address: your email address, e.g. [email protected]
Use SLL: tick this box
On the SMTP Authorisation tab on the Email Options dialog:
SMTP Server Username: your email address, e.g. [email protected]
SMTP Server Password: the password for your email account, e.g. for your [email protected] account.
If you need more help with all of this, just post back here, and one of us will oblige.