Perhaps these logs are related:
Apr 26 06:50:02 box postfix/master[17276]: daemon started – version 2.11.0, configuration /etc/postfix
Apr 26 06:50:02 imap-login: Info: Login: user=info@example.com.au, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=17331, TLS, session=<Vxoq3gNO0wB/AAAB>
Apr 26 06:50:02 imap(info@example.com.au): Info: Disconnected: Logged out in=15 out=422
Apr 26 06:50:04 box spampd[5829]: 2017/04/26-06:50:04 Server closing!
Apr 26 06:50:04 box spampd[5829]: Couldn’t unlink “/var/run/spampd.pid” [Permission denied]
Apr 26 06:50:08 box postfix/tlsmgr[17751]: open smtp TLS cache btree:/var/lib/postfix/smtp_scache
Apr 26 06:50:08 box postfix/tlsmgr[17751]: tlsmgr_cache_run_event: start TLS smtp session cache cleanup