Another thing you might try: change the port for outgoing mail. Port #25 was long the standard port for outgoing email but quite a few ISP's now use port 587. The port # appears on the same screen in OE email account settings as the outgoing server (your smtpmail.waitrose.com code).
Can you tell us a bit about waitrose.com? Are they your ISP or do you just use an email account with them? If they are supposed to be a full-featured ISP, they seem to do a poor job of providing information such as setting up a POP email account in Outlook Express or another email client. Such info seems non-existent on their website.
Have you considered other email options? Google's free email service, gmail, is quite good, and it can be used with Outlook Express or another email client.
If waitrose.com is not your ISP, if it's just an email account, then using an email account provided by your ISP is another option.