Email drop when using Outlook mobile app and email server is behind f5 ltm

I am having an email server behind ltm. I have an ltm box which is acting as a Link load balancer. I have configured a performance Layer4 virtual server on f5 ltm with a pool member of actual email server. The problem is users send an email using outlook mobile application, then the mail drops sometime and it gets stored in the draft folder but if i use any other email client in mobile, then their is no challenge, all mails pass through. Need to know what is the root cause?