All the settings are correct. It is the configuration of sylpheed, HELO is the start of the handshake and sylpheed passes certain parameters to freenetname, it is these parameters that are missing. I can't work out why.
(This is my first post to these forums - so please be gentle if I'm breaching local customs or anything!) Running the command 'hostname' on my CnMbook returns '(none)', which is what appears in the SMTP server's response to Sylpheed's HELO. Normally, the 'hostname' of the machine is stored in a file /etc/hosts - but for some reason, the CnMbook has no such file; instead it has 'hostname' which is where the name I gave the machine in 'Settings - Network setup' is stored.
Just to see what happens, I created a file /etc/hosts and in it I put the usual basic information (all on one line)
127.0.0.1 shortname longname localhost
(There's a tab after the IP number; shortname and longname are the same if you only give the machine a one-word name such as mylll or you can have a longname such as mylll.is.black; localhost is the generic name all computers have for referring to themselves internally).
Now when I send an email, the SMTP server shows my computer's name instead of '(none)'. That change seems to persist after a reboot too :))