{Only time for a quick reply now... get back to me if you need more}
Sounds like your Outlook isn't setup to use your ISP's mail server. For various reasons, mainly to due with spam prevention, many sites are being more rigorous about where they allow mail from (perhaps the site you are trying to email has recently changes ist policy.) You need to check that Outlook is correctly configured to use your ISP's mailserver. Don't have XP/Outlook handy, but look
for settings for SMTP or SMTP server and check that they are set to the values given by your ISP.
HTH,