How to disable AUTH PLAIN on FortiMail 5.2.4 build 447

[vietleanz_FTNT]

How to disable AUTH PLAIN on FortiMail 5.2.4 build 447

If telnet to FE and issue ehlo dmz.lab, see 250-AUTH LOGIN PLAIN

220 FEVM040000048646.localdomain ESMTP Smtpd; Tue, 17 Nov 2015 21:57:00 -0800
ehlo dmz.lab
250-FEVM040000048646.localdomain Hello [192.168.11.1], pleased to meet you
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-8BITMIME
250-SIZE 10485760
250-DSN
250-AUTH LOGIN PLAIN
250-STARTTLS
250-DELIVERBY
250 HELP

I couldn't find anything from docs but from some FortiCare, engineer advised to

config system mailsetting

 set smtp-auth disable

end

But this command remove all SMTP auth.

220 FEVM040000048646.localdomain ESMTP Smtpd; Tue, 17 Nov 2015 21:48:22 -0800
ehlo dmz.lab
250-FEVM040000048646.localdomain Hello [192.168.11.1], pleased to meet you
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-8BITMIME
250-SIZE 10485760
250-DSN
250-STARTTLS
250-DELIVERBY
250 HELP
auth login
503 5.3.3 AUTH not available
Connection to host lost.

Is this expected? or any way to just turn off AUTH PLAIN

Thanks

Viet