I have postifx configured as a simple mail forwarder. I have 4 virtual domains in /etc/postfix/virtual with about 10 virtual mailboxes on each which all forward to my business email address.
Now, I would also like to send FROM one of those addresses. My business email SMTP will not allow me to send "FROM" other email address than my default one, so I am forced to use my own SMTP server when sending FROM this certain address.
How do I configure postfix to allow authenticated users to also send (message relaying)? There must be a way to do it without adding the users on the system. I am also connecting from a non-local network (from the public internet) so I guess mynetworks cannot remain as it is now: only allowing loopback addresses.