This may be more than enough for some people, but it's important to keep in mind that these 15 GB are shared by your Gmail, Google Photos, and Google Drive. Google gives its users 15 GB of free account storage. It's a good habit to delete emails once you are done with them, but if it's already too late and your Gmail is nearing its capacity, listen up.
It's still sitting there, taking up precious storage space. Marking an email as 'Read' doesn't mean it's gone.