I have an email address in the pacbell.net domain and another in btinternet.com. Both of these ISPs use Yahoo mail to admininister their email. When I added pacbell.net it replaced the btinternet.com mailhost that was previously added. All of the hostnames/domains listed seem to apply to Pacbell/SBCGlobal/AT&T email domains, none to BT.

The Mailhost configuration only requires you to have a single entry per mail host. If the BT and PacBell mail both use identical routes then the Mailhost will simply replace the Email address but otherwise not add an additional entry. From your description I'd say this would be the normal and correct behaviour.

Andrew