About link mailboxes in hybrid, you could have a look about this article: EXCHANGE RESOURCE FOREST AND OFFICE 365 – PART IV
Personally, I suggest you merge the two forests, which will facilitate the management and deployment of hybrids：
- After adding domain B in forest A "Active Directory Domain and Trusts", you will could use domain B as logon name in forest A.
- Then, add the domain B as an accepted domain in forest A Exchange, you will could use domain B as email address for forest A mailbox.
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