Exchange Server mailbox permissions with Powershell

Send on Behalf – This will grant testuser user send on behalf permissions for Accounting mailbox:

 Set-Mailbox accounting -GrantSendOnBehalfTo testuser

Send As – This will grant testuser user send as permissions for Accounting mailbox:

Add-ADPermission accounting -ExtendedRights Send-As -user testuser

Full Mailbox Access – This will grant testuser user, full access to Accountng's Mailbox:

Add-MailboxPermission -Identity accounting -User testuser -AccessRights FullAccess -InheritanceType All