So that there is no misunderstanding, here is the current status. From the forum go out the info@example.COM and that should be forwarded to webmaster@example.NET (these are 2 different domains). The webmaster@example.NET is the admin of MIAB.
I sent the emails by means of Putty from the forum, so with
echo "relay test" | mail -s "relay test" firstname.lastname@example.org The email always came in as email@example.com and then I created the
sender_canonical_maps = hash:/etc/postfix/sender-canonical with the following content
firstname.lastname@example.org email@example.com so the right sender will be displayed as soon as I send a testmail via Putty, but if I make a forwarding in MIAB do so firstname.lastname@example.org to email@example.com, and get a testmail from the forum about Putty I as an error message (in the mail.log)
firstname.lastname@example.org: Sender address rejected: not owned by user. Where is the mistake? Thank you for your efforts.