.muttrc哪里设置错啦？

Question

请看 .muttrc

set folder="/home/debian8/Mail"
set editor="vim"
set sendmail="/usr/bin/msmtp"
set use_from=yes
set realname="xxxx"
set from=xxxx@xxxx.com
set envelope_from=yes
set mbox_type=mbox
set mbox="/home/debian8/Mail/inbox"
set spoolfile="/home/debian8/Mail/inbox"
set postponed="+postponed"
set record="+/sent/"
set check_new=yes

ls Mail
inbox  postponed  sent

发信后，报错
error info:
/home/debian8/Mail/sent/ is not a mailbox.
Could not send the message.
请问，哪里错了？