I want to give another user full access to an Amazon WorkMail mailbox.
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You can grant permissions to users to allow access to a shared mailbox without shared access credentials. The Full Access permission allows for full read and write access to the mailbox. You can also grant Send As or Send on Behalf permissions.
Note: You can grant the Full Access mailbox permission only to other available users. You can grant the Send As or Send on Behalf permissions to both users and groups.
For instructions on how to grant mailbox permissions, see Working with mailbox permissions.
After permissions are granted, the user can use the Amazon WorkMail web application to access the shared mailbox.