it is great- now it works!!
I am very satisfied with the help of your support, my problem is solved and everything works very well. The duplicate extension for Sieve filters does the job. A big compliment and many thanks for the great support!
I wanted to use the Option "Configuration: Email-> Messages-> Always send a copy (Bcc)..." This already works for outgoing mails (postfix will do this, right?). For incoming mails you have to add a line with
'redirect: copy "<catchall-mailbox>";' into "/var/lib/nethserver/sieve-scripts/before-sieve/".
And then it is usefull to install/ use the 'Duplicate extension' for Sieve filters to avoid duplicate mails.
The only thing I have to do is to create a custom template for sieve...
Finally I am very pleased to found NethServer Project and this community.