I have a client who is using Outlook 2013 on Windows 8, they have Google apps (Gmail). She is complaining that some people from different companies "sometimes" don't get her emails when using Outlook, no errors, no bounce backs.
However if she uses the browser she has no problems. Other people in the office can send and receive to people she is having problems with, so it's not blacklists or anything like that.
It happens whether it is a new email or if she responds to an email.