The bootmgr.efi file does not work when you try to boot via PXE with UEFI machines, and causes a failure PXE-E99.
UEFI computers booting via PXE.
Deployment Solution 8.0 Hotfix2 and lower.
A number of EFI files are extracted from the Windows Automated Installation Kit (WAIK) however not all of them can be used for PXE booting computers into EFI.
Bootmgr.efi is required for ISO images
Bootmgfw.efi can be used for PXE boot
Bootx64.efi should be used for 64-bit PXE boot of UEFI devices
BootIA32.efi should be used for 32-bit PXE boot of UEFI device
All the relevant EFI files are present in the BDC package  that is replicated to the PXE Servers in the environment however only the bootmgr.efi and bootmgw.efi are used in the SBS image directories .
When the PXE Service is started it creates a list of EFI files and stores a reference into memory, however it only stores a reference to the latest EFI file found. Because the file loading is done in alphabetical order, it means bootmgfw.efi is referenced before bootmgr.efi. Thus bootmgr.efi will always be used.
 PackageId = '374E1C49-4F58-4F5C-8D51-07A30F0D44AD', Package Location = '\\%ComputerName%\NSCap\bin\Deployment\BDC'on the SMP
 '%ProgramFiles%\Altiris\Altiris Agent\Agents\Deployment\SBS\Images\<Name of your PXE environment>\<Architecture>' where <Name of your PXE environment> is the name you have specified when creating the pre-boot environment and <Architecture> is x64 or x86.
Upgrade to 8.0 HF3 or later
To work around this issue you will need to edit each of your PXE automation environments by going to "%ProgramFiles%\Altiris\Altiris Agent\Agents\Deployment\SBS\Images\<Name of your PXE environment>\<Architecture>":
or add the bootx64.efi (for 64-bit PXE boot images) from the local BDC package local copy 
or bootia32.efi (for 32-bit PXE boot images) from the local BDC package local copy 
Restart the PXE Service
Note! This change will have to be made every time you rebuild the PXE server, because the rebuild will return all files back to the original default versions.
For the changes to remain after recreating the SBS boot images you should rename bootmgfw.efi to bootmgr.efi from the BDC package .