Email going to spam or not delivered at all

One of my employee's emails will either not be delivered or go into spam for some (but not all) of her recipients. Some of her recipients will get emails without a problem. When she sends to 1 of my accounts, the emails just disappear (her email says that it was sent). When she sends it to my other email account, it always ends up in spam. This happens to maybe 15% of the people that she sends to.