Hi.
I do not have folder /var/spool/postfix/public/pickup while i use MailPlus-Server.
Tried your trick with (I guess) equvelent folder:
root@NAS01:~# mkfifo /var/spool/@MailPlus-Server/postfix/public/pickup
mkfifo: cannot create fifo ‘/var/spool/@MailPlus-Server/postfix/public/pickup’: File exists
So I am still at the same place