Finally found someone prepared to listen at Vodafone.....
It seems the problem is with the default e-mail outgoing mail server. Not all normal ISP servers will accept the Vodafone data stream, so in OE6 one must create a second account with server.vodafone.net as the outgoing mail server. Then assign the correct dial-up method to each account (landline via normal access code, VMC card via the VMC Internet access) and define the default......
A bit complicated....why? Surely the installation software should have been able to do all this with transparency to the customer?
But at least it now works!