this is for information only since it is wrongly written in the manual of ISPconfig3.
The manual says it shoud be like:
: <ISPconfig server>
: <email@example.com or domain.tld>
Actually. If you want to be a backup MX for a whole domain. You should use "@domain.tld
". If the @ is not included it will mention "rejected: User unknown in virtual mailbox table;
the main.cf in /etc/postfix should have:
transport_maps = mysql:/etc/postfix/mysql-virtual_transports.cf
relay_recipient_maps = mysql:/etc/postfix/mysql-virtual_relayrecipientmaps.cf
good luck everyone searching for this. I hope this gets rectified in the manual.