Working now...

I enabled port 24 in dovecot on both proxy and backend (previously i was just telling postfix to listen to port 24 on the backend server) -- Confirmed lmtp on 24 was working on both servers by telnet localhost 24 (220 Dovecot LMTP ready response)

here are my settings

proxy is using inet for LMTP

dovecot/conf/10-master.conf service lmtp { unix_listener lmtp { #mode = 0666 }

Create inet listener only if you can't use the above UNIX socket

inet_listener lmtp { port = 24 } }

postfix/main.cf

virtual_transport = lmtp:localhost mailbox_transport = lmtp:localhost

backend is also inet (to receive) and unix socket (to deliver locally)

dovecot/conf/10-master.conf service lmtp { unix_listener lmtp { #mode = 0666 }

unix_listener /var/spool/postfix/private/dovecot-lmtp { group = postfix

mode = 0660

user = postfix }

Create inet listener only if you can't use the above UNIX socket

inet_listener lmtp { port = 24 } }

postfix/main.cf virtual_transport = lmtp:unix:private/dovecot-lmtp mailbox_transport = lmtp:unix:private/dovecot-lmtp

postfix/master.cf removed: 24 inet n - n - - lmtp