relay other domain based on email user

[vincent li <vincentinsz yahoo com>]

hi folks:
 i only want relay other domain based on email
account, for example, email user A iaspec com can send
email to any domain on my email server; user
B iaspec com can only exchange email within domain
iaspec.com,and can not send mail to any other domain.
how could i reach this goal, please give me a hand...

thanks in advance 

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