Okay, now I got to a solution that works:
If I
-
add all configured mail accounts to the
20accounts
section of/etc/postfix/virtual
and do apostmap /etc/postfix/virtual
as described here (this should no more be necessary when the corresponding bug is fixed); -
use
sendmail
instead ofdovecot-lda
in the getmail configuration as suggested by @filippo_carletti, see second post.
then all works as desired: E-Mail from the internet to existent users is delivered to their accounts and E-Mail to non-existent users is delivered to the account defined in “Accept unknown recipients”.
Thank you @filippo_carletti and @davidep for your support!
Davide, can you please explain your concerns with Filippos suggestion to use sendmail?