The installer ( PI OfficeMail(TM).exe) does not have MS certificate and comes up as unknown publisher.
The PI OfficeMail(TM).exe is not signed properly and coming up as unknown publisher when executed.
PI OfficeMail(TM).exe is just a wrapper for signed installer.
PI OfficeMail(TM).exe contains piom.exe (PIOM.EXE is signed) + several other files:
If you need the installer which has the certificate use PIOM.EXE, or if you are deploying the driver via Active Domain, use the MSI installer.
Otherwise the PI OfficeMail(TM).exe is still suitable installer.
UPDATED: May 16, 2017