SMTP Authentication failure and mobile users cannot send or receive mails

[monney1]

Please i need your help on this. I have MS Exchange 2010 as my main email server. I have configured FortiMail for my network in gateway Mode behind the firewall, Fortigate.

There's an Integration of Exchange 2010 Edge Transport with Forefront Threat Management Gateway (TMG) 2010 so the issue i'm having is the legitimate outgoing mails by internal domain users are detected as spam and classified as smtp authentication failure and the direction is given as unknown.

Outlook works for desktop for incoming and outgoing mails but Mobile devices of internal domain users are unable to send and receive mails.

Mobile devices are using Exchange Autodiscover for their settings.