Easiest way is with Macromedia Dreamweaver Ultradev, which will generate all the HTML stuff for you and some of the VBScript as well, it is available for a 30day trial...

The mail client is MAPI, when I send it through the client eg Outlook it works fine,

The stored procedure is

CREATE PROCEDURE emailme AS

exec master.dbo.xp_sendmail

@recipients='[x400:c=gb;p=nhs national int;a=nhs;dda\:rfc-822=aslam.atcha(a)bury-roch-ha.nwest.nhs.uk] ',

@type='X400',

@message="test",

@subject="test"

GO

And the error message...