I was also checking the /var/logs/maillog file and it could be related to spamassassin (please note I've replaced the servername with ???? and message id with .....):
Dec 5 00:42:45 server spamd[25527]: spamd: connection from localhost.localdomain [127.0.0.1]:37726 to port 783, fd 5
Dec 5 00:42:45 server spamd[25527]: spamd: handle_user (getpwnam) unable to find user: 'spamd'
Dec 5 00:42:45 server spamd[25527]: spamd: still running as root: user not specified with -u, not found, or set to root, falling back to nobody
Dec 5 00:42:45 server spamd[25527]: spamd: checking message <
[email protected]> for spamd:99
Dec 5 00:42:45 server spamd[25527]: plugin: eval failed: bayes: (in learn) locker: safe_lock: cannot create tmp lockfile /.spamassassin/bayes.lock.server.?????????.com.25527 for /.spamassassin/bayes.lock: No such file or directory
Dec 5 00:42:45 server spamd[25527]: spamd: clean message (-0.1/5.0) for spamd:99 in 0.3 seconds, 2759 bytes.
Dec 5 00:42:45 server spamd[25527]: spamd: result: . 0 - DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FROM,HTML_MESSAGE,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,SPF_PASS scantime=0.3,size=2759,user=spamd,uid=99,required_score=5.0,rhost=localhost.localdomain,raddr=127.0.0.1,rport=37726,mid=<
[email protected]>,autolearn=unavailable autolearn_force=no
Dec 5 00:42:45 server spamd[25526]: prefork: child states: II