A Microsoft file hosting and synchronization service.
Only answer I can think of right now is that they're being greedy. I just got the same issue, was wondering why I hadn't received emails for over a week. 2.4GB/15GB Email storage used but I can't send or receive emails because my Microsoft storage is at 5.1GB/5GB used when email attachments shouldn't count towards Microsoft storage or OneDrive but it seems like they don't give you a choice. Literally no point of having the separate Email storage then. This is such nonsense, it all seems kind of shady right now as if to make people buy extra storage. I'm gonna move to Gmail at some point if they keep this up.
One solution that may work well depending on your needs is to go to your OneDrive > My files, then select any files with a large size that shouldn't have been backed up to OneDrive but was auto selected, and delete it from OneDrive. I had a 4.4 GB file that I didn't need backed up at all but it was auto selected and took up almost all of the 5GB of Microsoft storage.