Showing Old display name on mail


New Email
In my organization, some mail id is fixed. when change person, just change mail display name only. My problem is.. still older or oldest mail display name showing after changing mail display name.

For example, umesh is president, his mail id in mail configuration there is display name is Umesh. After leaving umesh, came new employee Ayush. and his mail id is same also configure mail id with display name Ayush. update my contact name ayush. but when came mail from president, showing umesh or older person name. Anyone can suggest for best solution. thanks in advanced.