Cannot access mail after password change

iPhone 3G

My main e-mail account is BTinternet.com (joined with Yahoo). Recently they updated their mail system, I had to create a new account and change my password. Then my e-mail (accessed on my Mac computer) did not work for a while. When it came on again, I was made to change my password again. I did so. Now I can send and receive mail on my computer again, both directly from the BT/Yahoo mail page and from amail client on my computer.

I then very carefully changed the server settings on my iPhone and changed the password there (for incoming and outgoing mail) to the latest version. But now I cannot send or receive mail on the iPhone. I have checked and rechecked all settings, but all I can get is a message that my username and/or password are wrong. But they are not wrong. I have repeatedly re-entered the right version. What can I do?

Thank you for any help anyone can offer.