Why won't our emails go through?

From my work email, I used to be able to email my friend's work email. All of a sudden, she stopped getting them. Then I emailed her from my hotmail. She was getting those for a few days, then wasn't able to get those either. We checked, and we are not blocking each other. I can receive her emails. My IT dept. said that it is not on my end because they are showing the emails as delivered. Any ideas?