offcs - thanks for the advice.

It's odd that some btinternet email addresses receive my emails OK but not others, so I don't think the domain can be blocked.

Any idea how/who to contact at btinternet? Aren't they tied up with Yahoo?

When you say "btinternet", do you mean just btinternet.com, or also btconnect and there's another one as well that I forget?

I'm just wondering if emails to one of them are getting blocked but the others are OK.

They are all [email protected]

Has your domain got a SPF record?

I don't think it's possible to do it with 1and1. From their website -

There is currently a proposal that would add text records (SPF) to a domain�s DNS that will contain the mail servers that are permitted to send mail from that domain. The incoming mail server will then reference these records, and either accept the mail or deny it based on the SPF record. This will require that both the host of the domain�s DNS and the receiving mail server have this type of protection implemented. It may be some time before the community as a whole adapts to using this policy or a similar one. There is currently no implementation of these policies planned for 1&1 domains.

Try this
http://help.yahoo.com/l/us/yahoo/mail/postmaster/def...