When you get an 0x800-type error in Outlook or Outlook Express, you can check the error in the Microsoft Support Database (http://support.microsoft.com), which will give you the cause and the solution to the error.
Some of the common causes of these errors include:
SMTP authentication enabled or disabled when it should be the opposite
Try switching between the two settings and see if either works.
Your ISP blocking access to third-party SMTP servers
Try changing the SMTP port to 587 or using your ISP’s SMTP server details.
Your Outlook account becoming corrupt
Try deleting and recreating the account.
The hosting package and/or domain name have only been created in the last few hours
Wait for the DNS changes to fully propagate.
Your firewall blocking your mail client
If you have not used this client before, see if it is having a problem with your firewall settings.