Send mail failed

[zxconan]

Hi,

nodebb version 1.8.1
os ubuntu 16.04

when send confirm mail to new users,
output in nodebb log:

io: 1 on [ { type: 2, nsp: '/', id: 1, data: [ 'user.getUnreadCounts' ] } ]
io: 1 on [ { type: 2, nsp: '/', id: 2, data: [ 'user.emailConfirm', {} ] } ]
2018-04-04T06:02:18.196Z [2448] - error: Error: Sendmail exited with code 67
at ChildProcess.sendmail.once.code (/home/ubuntu/nodebb/nodebb/node_modules/nodemailer/lib/sendmail-transport/index.js:137:27)
at Object.onceWrapper (events.js:272:13)
at ChildProcess.emit (events.js:180:13)
at Process.ChildProcess._handle.onexit (internal/child_process.js:209:12)

Thank you.