[mrtg] postfix mail account problem

[bazzar]

dear all 
i have problem on my postfix configuration. the mail problem is when any
user on my mail system sent message to another user name he can use faked
user name. 
for example user name x at mydomain.com can send message to me and when i open
the message, the sender name displayed as y at mydomain.com, where y user name
is not exists (fake name) 

how i can stop this 

please any help 
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