Mail to Username - Domain instead of Hostname

[debreuer]

Hi all,

the users want to mail a other user without type the complete mail adress.
Example:
The Server has a Account User1 and User2 with the domain: domain.com

user1 want to write an email to user2 with email-adress: "user2"

the server send the email to user2@mail.domain.com

can i change that the server send the email to user2@domain.com instead of @mail.domain.com???