postfix: connect() to private/proxymap by smtpd fails with ECONNREFUSED

Issue

connect() to private/proxymap by smtpd fails with ECONNREFUSED continuously.

postfix/master[11921]: daemon started -- version 2.10.1, configuration /etc/postfix
postfix/smtpd[11986]: warning: connect #1 to subsystem private/proxymap: Connection refused
[...]
postfix/smtpd[11986]: warning: connect #10 to subsystem private/proxymap: Connection refused
postfix/smtpd[11986]: fatal: connect #11 to subsystem private/proxymap: Connection refused
postfix/master[1807]: warning: process /usr/libexec/postfix/smtpd pid 13986 exit status 1
postfix/master[1807]: warning: /usr/libexec/postfix/smtpd: bad command startup -- throttling
postfix/smtpd[12624]: warning: connect #1 to subsystem private/proxymap: Connection refused
postfix/smtpd[12624]: warning: connect #2 to subsystem private/proxymap: Connection refused

According to the "man connect", connect() fails with ECONNREFUSED if "No-one listening on the remote address". But strace and netstat shows that postfix master process listens private/proxymap. So we can't understand why connect() fails with ECONNREFUSED.