First convert the linked mailbox to user mailbox using this command:
Set-User -Identity <e-mail address> -LinkedMasterAccount $null
and then convert user mailbox:
Set-Mailbox <Mailbox Name> -Type shared
To give the appropriate permissions to the user
Add-MailboxPermission <Mailbox Name> -User "<domain\username>" -AccessRights FullAccess
Add-ADPermission Research -User "<domain\username>" -ExtendedRights Send-As
To make the sent email to be available in shared mailbox sent items:
For emails Sent As the shared mailbox:
Set-Mailbox <Mailbox Name> -MessageCopyForSentAsEnabled $True
For emails Sent On Behalf of the shared mailbox:
Set-Mailbox <Mailbox Name> -MessageCopyForSendOnBehalfEnabled $True
