Thanks but that's not practical as they've had their email addresses for 10 years or more so don't want another email address. As Tesco is a major ISP in the UK it could well be affecting a lot of other people too - I have 6 friends who might sign up but won't unless the problem is fixed for the reason given above. I've seen this type of problem before and all that was required is to update a look-up table in the app that is used to validate ISP domain names.