[Dovecot] unclear alias behaviour

[andel]

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 dovecot accounts, 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