go to add remove program on the problem machine
select symantec endpoint >change >modify follow the wizard >custom >disable outlook scanner
follow the wiz reboot the problem machine and try sending invitation
Above is for Client computer
If you have sep manager installed then go to policies tab >antivirus and spyware policy >edit the policy which is applied in the network
look for out look protect uncheck all the option
and then assign this
i would suggest you to try this on few machines first and then implement in the network if it works
all the best