Many customers are now finding that their ISP is blocking port 25 (the traditional outgoing email port) to any other provider except their own. This means, that whilst your ISP is being reactive about the amount of spam on their networks, i.e. stopping compromised machines blasting out spam, they are essentially forcing you to use [...]

It looks like after approximately 3 weeks BT/Yahoo have actually worked out what greylisting is meant to be and have applied this to their servers.
Dec 20 06:58:18 cobalt qmail: 1198133898.559094 starting delivery 1113: msg 7375264 to remote ahiddenemail@btinternet.com
Dec 20 06:58:19 cobalt qmail: 1198133899.434814 delivery 1113: deferral: Sorry,_I_wasn’t_able_to_establish_an_SMTP_connection._(#4.4.1)/ (this message varies from the typical deferred to [...]