Email IDs are placed under 'example@domain.com' for couple of users?

Hi,

We use exchange 2013 CU5 hybrid of O365

We observed for couple of users like - when they receive email ID from external IDs it will place the email under 'example@doamin.com' format rather than usual way ofUserName/Domain/Department display name format

Eg:

Usually if any email from external IDs to internal user the email once received, will show as display name format -UserName/Domain/Department

But today couple of users complained saying their external email user complained , they see as -'username@domain.com'

what could be the reason

Manju Gowda