Email address, and not username, as default "From:" address

[ipslave]

I have one server running Horde as a webmail client frontend, and another server running ISPConfig. Both run Debian Etch installed using the perfect guide. System is configured to allow 'username@domain.name' logins.

Scenario;
Create a user named John Doe, with login 'domain.name_johnd' and email addresss 'john.doe@domain.name'.

Problem;
What I've found with this is that with Horde (and any other webmail client I set up on the server for that matter) the default From: field becomes 'johnd@domain.name' and not the email address specified when the account was created. This can be changed by the user, but is there a way to automate or default this process with ISPConfig?

I know that one could simply specify the username as domain.name_john.doe, but this means users having longer usernames to login with - which I am hoping to avoid...