As described in detail in thread
https://forums.opensuse.org/showthread.php/522810-problem-starting-amavis, if
amavis and amavis-milter are started at boot via the bundled amavis.service
unit file it appears that:

1) amavis-milter starts (/var/log/mail) 
2) it creates the socket /var/spool/amavis/amavisd-milter.sock
3) it suddenly and silently dies (ps does not report it any more)
4) amavis starts and creates its own socket/var/spool/amavis/amavisd.sock
5) when a mail comes there is a connection refused message on 
   amavisd-milter.sock (which netstat -na shows NOT to be in listen status)

Our current workaround has been NOT to use the ExecStartPre and ExecStart
mechanism, but to force starting amavis-milter and amavis in sequence in a
single shell script (if they are started in sequence manually or in a shell
script both remains running and everything works ok)