Mail account already exists!

[kilburn]

There is a configuration variable (login as admin -> settings) that specifies if these default mail accounts have to be automatically created. If it's enabled, these accounts are created automatically, pointing to the user's e-mail address (as specified when creating the user), and do not appear in the panel to prevent the user from deleting them.

You might try changing the user's e-mail, but I'm not sure if this will also change the mail forwards :S Another alternative is to directly edit the postfix map (/etc/postfix/ispcp/aliases -and- /etc/ispcp/postfix/working/aliases). Don't forget to postmap the former one so that postfix actually applies the changes.