I'm trying to get ClamAV and clamsmtp to work on my Linux debian system. I have made the following configurations;

/etc/postfix/main.cf:
content_filter = scan:127.0.0.1:10025
receive_override_options = no_address_mappings

/etc/postfix/master.cf:
scan unix - - n - 16 smtp
-o smtp_send_xforward_command=yes

127.0.0.1:10026 inet n - n - 16 smtpd
-o content_filter=
- o receive_override_options=no_unknown_recipient_chec ks,no_header_body_checks
-o smtpd_helo_restrictions=
-o smtpd_client_restrictions=
-o smtpd_sender_restrictions=
-o smtpd_recipient_restrictions=permit_mynetworks,rej ect
-o mynetworks_style=host
-o smtpd_authorized_xforward_hosts=127.0.0.0/8

/etc/clamsmtpd.conf:
OutAddress: 10026
Header: X-AV Checked

....but I don't get " X-AV Checked" added to the header of the mails. Why??

I don't know if it's because in your example it adds an empty header (doesn't it log errors somewhere?), could try a "Header: X-AV Checked: Yes" line.