Iphone 6 exchange email problem

one of the user using Iphone6 to sync exchange email account few month already.

But he told me, start from this week when he want to delete some email in Iphone, it work fine.

after few hours, the deleted Item will go back to Inbox...

we are using Exchange server 2013, many user using Iphone to get email, but no this problem.

anyone have idea?