lmtp proxy 'Invalid FROM: Missing domain'

[Marc Roos]

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.