Can not use sendmail for email notification

I am running Gitea on Ubuntu 16.04 and trying to enable email notification. In app.ini I configured it to use sendmail by setting the mailer section as

[mailer]
ENABLED       = true
FROM          = gitea@ourdomain.com
MAILER_TYPE   = sendmail
SENDMAIL_PATH = /usr/sbin/sendmail

But when I clicked on the “Send Testing Email” on the Site Administration > Configuration page, I got the following error message:

Failed to send a testing email to 'xxx@ourdomain.com': gomail: could not send email 1: fork/exec /usr/sbin/sendmail: no such file or directory

I’ve verified that /usr/sbin/sendmail exists and is working fine. Does anyone have suggestion on what might have caused this error?