lmtp proxy 'Invalid FROM: Missing domain'

Looks like I have a correct working proxy on pop3. On both backend servers I can run

cat /tmp/test.msg | /usr/libexec/dovecot/lmtp

Giving 250 2.1.5 OK 354 OK Info: Disconnect from local: Connection closed (in DATA)

However if I run the same command on the proxy, I am getting this error 'Invalid FROM: Missing domain'. How is this FROM even relevant if message just needs to be delivered to RCPT TO.