I bought a new server last Friday, spent part of the weekend installing it and migrating all of my email/websites to it. Everything went pretty smoothly, but I did have one hiccup with my Postfix/Dovecot LDA setup. When trying to deliver a message, I would get this sequence in my maillogs:

Jun  9 12:39:10 www2 postfix/pipe[24300]: fatal: user= command-line attribute specifies mail system maildrop group id 126
Jun  9 12:39:11 www2 postfix/qmgr[24159]: warning: private/dovecot socket: malformed response
Jun  9 12:39:11 www2 postfix/qmgr[24159]: warning: transport dovecot failure -- see a previous warning/fatal/panic logfile record for the problem description
Jun  9 12:39:11 www2 postfix/master[13698]: warning: process /usr/local/libexec/postfix/pipe pid 24301 exit status 1
Jun  9 12:39:11 www2 postfix/master[13698]: warning: /usr/local/libexec/postfix/pipe: bad command startup -- throttling

At some point, postfix must have stopped allowing stuff to be sent through the 'pipe' command running as whatever you have in setgid_group. My dovecot line in master.cf previously looked like

dovecot   unix  -       n       n       -       -       pipe
  flags=DRhu user=dovecot:maildrop argv=/usr/local/libexec/dovecot/deliver -d ${recipient}

I fixed this by dropping the group:

dovecot   unix  -       n       n       -       -       pipe
  flags=DRhu user=dovecot argv=/usr/local/libexec/dovecot/deliver -d ${recipient}

Works fine.