Make sure that you know the SMTP server address used by your mail service. If you send your mail via one of the UK's main ISPs it will be on one, or both, of these pages:
http://www.idl.uk.com/email/isp_settings.htm
http://www.kitz.co.uk/isp/dns.htm
Otherwise you can find it on the support pages of your email provider's website.
Go to Tools > Accounts.
Click on the 'Mail' tab.
If you've only got one mail account it will probably already be highlighted. Otherwise click on the relevant account to highlight it.
Click 'Properties'.
Click the 'Servers' tab.
The information in the Outgoing (SMTP) field is probably incorrect.
Correct it.
Click 'Apply', 'OK' and 'Close'.
If the information in the SMTP field appears to be correct, simply delete your mail account and then create it again. To do so, start as above but select 'Remove' instead of 'Properties'. Click 'Yes' when asked to confirm that you want to delete the account. Then click 'Add > Mail' and follow the instructions to re-create the account. (You'll need to know your username, password, SMTP and POP3 server addresses).
Chris