In Debian, Postfix's smtp(8) uses a different name to avoid naming conflicts.
Need to point lmtp(8) to the right file.  See #920356.
Index: postfix-dev/man/man8/lmtp.8
===================================================================
--- postfix-dev.orig/man/man8/lmtp.8	2015-01-29 17:21:00.000000000 -0500
+++ postfix-dev/man/man8/lmtp.8	2019-02-16 02:46:57.254292885 -0500
@@ -1 +1 @@
-.so man8/smtp.8
+.so man8/smtp.8postfix