You need to modify the PR_SENDER_NAME MAPI property (DASL name http://schemas.microsoft.com/mapi/proptag/0x0C1A001F). It corresponds to MailItem.SenderName in the Outlook Object Model, but it is read-only. MailItem.PropertyAccessor.SetProperty will not
let you modify it - you can do that only using Extended MAPI (C++ or Delphi) or Redemption.