Right, I need to access the collective consciousness again...

 Avast 5 is just out, and incorporates  a secure connection to the mail server.  Thus, you have TSL off in the mail client (Thunderbird [TB] in my case), and the relevant bit of Avast mail shield pointing at the SMTP server set to TSL, and it seems to work. (NB: I found somewhere else on here that you need to use the full username, ZEN456789@zen.co.uk as the short form doesn't work for this).

 Thing is, if you have the username specified in TB SMTP settings, you get bugged for the password not only when you start up TB, but the first time you send an email.  you can make Password Manager remember it, but I'm not greatly keen on that.  If you don't specify a username in TB, then you don't get the extra buggage, and the mail connection works anyway.

 Is there a security implication to not specifying the username in the SMTP setting for the mail client, i.e. is this dangerous in some way?  How exactly should I set up the smtp settings?

 In my defence there isn't a standard 'how-to' for TB and email (maybe it's time there was one as TB is popular now).