unclear alias behaviour

Hi all

In /etc/aliases I have set up an alias for "root" pointing to "david"
which is a unix account.
Other email accounts are virtual using dovecot, one of them is "andel".
They all work fine using /etc/postfix/virtual when used with the domain
name which is defined in /etc/postfix/virtual.domains.

But the alias "david andel" in /etc/postfix/virtual does work only when
sending to "david <at> domain.com" but not for the local mails sent to "root".

And then there is another behaviour I don't understand. In my virtual
mail folder /srv/vmail/domain.com dovecot made a new directory structure
"david" which I didn't intend, for postfix should rewrite it to "andel".

Where is the error?

Thanks for any hints,
David