Go to Tools>Accounts and click on the 'Mail' tab.
If your Tiscali account is the only one you've got, it will probably already be highlighted in blue. (If not, click on it to highlight it).
Then click 'Properties' and the 'Servers' tab.
Check that the following information is correctly shown:
The incoming mail server should be pop.tiscali.co.uk
The outgoing mail server should be smtp.tiscali.co.uk
The account name should be your FULL email address (NOT just the bit before the @ sign)
Delete and retype your password.
'Remember password' should be the ONLY box with a tick in it.
Click 'Apply', 'OK' and 'Close'.
Try sending and receiving mail. (Send an email to your own address)
If that doesn't work, start as above but (instead of clicking on 'Properties') click on 'Remove' and 'Yes'. (Don't worry. You're only deleting your account details. Existing mail, in your Inbox for example, won't disappear). The click on Add>Mail to recreate your account, using the information as above, which is summarised here:
http://tinyurl.com/22jz7yk
Chris