How to get user's email in multi-domain?

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Exchange-Server

Hi Guys,

I encountered a issue when trying to get the email address of the Exchange user in multi-domain environment.

Scenario:
1.user1 is in DomainA; its email like user1@DomainA.com
2.There is another user in DomainB with the same name as user1 in DomainA, whose email would be user1@DomainB.com
3.DomainA and DomainB share the same Exchange Server
4.Somehow, I can fetch the user's EntryId via MAPI

Question:
how can i get the email address of the user by knowing the user's EntryId?

Thanks,
DragonFly