from address of internal email account blank on on mobile

Hi

for some reason i cannot find why, only 2 user when they send mail internally it appear as  there is no sender address , "from" is blank but only on mobile phone platforms.

on regular outlook interface i can see the sender from address and display name  

if the user send to gmail account the from address and display name is therenormally.

i tried to check x400 adrress or replace it. create a new oab. non of this helps.

please advice,

thanks,

lior latin.