Thanks for all the suggestions. I thought I should let people know that the problem is solved. Its a bug, but
not an SME bug.
Went back on site this morning and tried several things without any change - any time I tried to send email, I got 'relaying denied'.
I then tried firing up Thunderbird on my laptop and let it autodetect settings. Which worked - once it was finished, there was no problem sending email. So the problem seems to be a Thunderbird one - even though all the settings were identical, the existing clients would still come up with the rellaying delayed error.
I then tried deleting existing profiles (after saving the address books). That didn't work (at least, not under Windows - Thunderbird didn't seem to be able to recreate profile information. So I had to uninstall Thunderbird and do a clean reinstall. At which point I was able to use its wizard to create new accounts, all of which worked. Problem solved, though it was a lot of work to fix....
A couple of observations, since they don't match the information in this link:
http://wiki.contribs.org/Email_-_Setting_up_E-mail_clients_for_SME_8.0That link suggests that the fully qualified server name needs to be used, but the Thunderbird wizard set the server names as 'mail.xxxx.col.uk'. And the encryption is working. I should also note that the wizard setup used port 25 for the outgoing server.
Hope this helps someone else....
Paul.