The Riva connection account for Exchange requires "Delegate" full access permissions to the mailboxes of the target users. For on-premises Exchange systems, this is normally configured with Exchange Management Console. For more information, see Riva for Exchange: Understanding Full Access permissions, Delegated Access, and impersonation.
For hosted Exchange services, assigning "Delegate" full access permissions is done using the online Mailbox management management console. Each hosted service uses a similar approach but the management console may differ in appearance.
With hosted Exchange services, the Riva connection account should be an administrator-level account that is granted "Delegate" full access permissions to each target user.
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