Where did you get the Sky email settings from? They decommissioned their old SMTP server in February. If you've entered 'smtp.sky.com', it will no longer work.
Review your account settings and check that this is what you've got:
Incoming mail server should be set to 'POP3' and defined as 'pop.tools.sky.com'. (NB: All lower case)
Outgoing (SMTP) server should show 'smtp.tools.sky.com' (all lower case).
The account name should show only the first part of your email address (i.e. just the bit before the '@' sign), not your full address.
Your password should be whatever you've decided, but remember that it's case sensitive. ('Password' is not the same as 'password').
Now, here's the bit that most people get wrong! Unlike many email services provided by ISPs, Sky accounts seem to require server authentication. Check that there's a tick alongside the box labelled 'My Server Requires Authentication'.
Chris