Jerry Nicholls:With cPanel you need to use POP-before-SMTP. ie if you login to the POP3 server before trying to send email it ought to work. There's a time window in which any IP address that successfully logs in to POP3 can then send email via SMTP.
Thanks Jerry - that's what the Outlook account is doing - it connects to the POP server and collects email fine, it just fails to connect to the SMTP server to send email.
I've had another look at the knowledge base article for email setup on cpanel accounts and wondered if it is related to me using the domain name I have pointed at the cpanel account rather than the subdomain.myzen.co.uk - I'm hoping, however, that this is a "Red Herring!"
Strangely enough the same account works fine when they're in the office using the Zen provided broadband connection. Hmmm.