When upgrading Postfix, I have this same problem. Upgrading process modified my "master.cf" from "lmtp unix - - - - - lmtp" to "lmtp unix - - y - - smtp".
I change again smtp -> lmtp : "lmtp unix - - y - - lmtp" and all began to work.
When upgrading Postfix, I have this same problem. Upgrading process modified my "master.cf" from "lmtp unix - - - - - lmtp" to "lmtp unix - - y - - smtp".
I change again smtp -> lmtp : "lmtp unix - - y - - lmtp" and all began to work.