To remove all lines that begin with a # and then delete all blank lines
  sed '/^#/d; /^$/d' dovecot-ldap.conf 

  sed -r '/^#/d; /^ +#/d; /^$/d; s/^/  /' dovecot.conf

 # insert 5 blank spaces at beginning of each line (make page offset)
 sed 's/^/     /'



http://www.eng.cam.ac.uk/help/tpl/unix/sed.html