tail -f /var/log/mail/errors
One more mystery solved was why nmap was showing port 25 as closed - postfix was listening only on the loopback interface - localhost.
inet_interfaces = allsolved that.
First I thought just copying the main.cf from the older server was the reason. But even replacing it with the main.cf.dist with appropriate modifications didn't solve the problem.
The first one was
fatal: connect #11 to subsystem private/proxymap: Connection refused
and the other, later, was
fatal: connect #11 to subsystem private/scache: Connection refused
Deleting the proxymap socket (present in /var/spool/postfix/private ) only changed the error to file not found.
mksock gave file not found - the mksock util was not in the system at least with that name...
proxy_read_maps = $local_recipient_maps
local_recipient_maps = unix:passwd.byname
removed the first error and gave the second one. This one was finally removed by later adding
smtp_connection_cache_on_demand = noOne more thing I'd overlooked was the differing postfix versions needing different commands to update the alias database and the virtual database. This is actually mentioned in the small text at the bottom of the linuxquestions link in small print. Anyway, for the record, they are