Email is not a guaranteed delivery protocol. Messages get dropped if any server in the path has a problem. With 80+% of email being spam, it's also possible that an aggressive filter on your ISP or an intermediate machine is selecting the messages incorrectly. Treating email as a reliable protocol does not make it so.
If it's all in-house Google, there's no reason why they can't develop some sort of guaranteed delivery protocol, just for internal use. That would be a huge boon to a calendaring app with notification.
It may not be guaranteed, but how many emails do you have dropped every day? If someone is missing an a day from some source, I'd think that its not a problem with the servers along the way but a problem with the source or destination.
