一直忘记master.cf里面的${extension}代表的意思，今天又再找一次，记录在下方

An example using address extensions (ie user+extension@domain.com (don’t forget to define the proper recipient_delimiter in Postfix’s main.cf)) to deliver to the folder ‘extension’ in your maildir (If you wish to preserve the case of ${extension}, remove the ‘hu’ flags, and be sure to utilize Modifiers in your dovecot.conf for mail locations and other configuration parameters that are expecting lower case):

dovecot unix    -       n       n       -       -      pipe  flags=DRhu user=vmail:vmail argv=/usr/local/libexec/dovecot/deliver -f ${sender} -d ${user}@${nexthop} -n -m ${extension}