[Nige]

Thanks both!
My thoughts are that Yahoo doesn't know it's a**e from its elbow, and the more I thought about it, the more I think its last answer is totally wrong.

Delay reason: SMTP error from remote mail server after initial connection:
host mx2.bt.mail.yahoo.com [217.12.12.192]: 421 Message from (91.186.0.56) temporarily deferred - 4.16.50. Please refer to http://help.yahoo.com/help/us/mail/defer/defer-06.html

Which server is the error message from there? - Yahoo's I believe. I also think that neither Viking nor Yahoo servers require BOTH SSL and SPA. Additionally, and I've only just spotted this (because I have Office XP on my wife's machine) I'm using Outlook 2003 at home and Thunderbird Portable when I'm travelling. Additionally, mail fails to Yahoo servers when I'm using webmail. Oddly, some messages do get through sometimes - one arrived today after 4 days.
Needless to say, Yahoo didn't actually ask me what client I was using.
As I said before, I'm not going down the full path of BT's Kafka-esque (Monty Python, if you prefer) bureacratic madness - A few more shots, then I give up. Friends, family, clients and anyone who'll listen will be encouraged to switch from any BT related mail soonest.