We are all guilty of saving too much of our email out of fear we may need the information they contain someday. That will cause your mailbox to fill up quickly if you are not diligent in deleting the email you do not read or use. Still, the amount of space available to us is shrinking with every email we store in a folder or archive for later use.
When you decide to delete the hundreds or even thousands of emails some providers will only allow you to delete what is present on the screen. To delete batches of email in a Gmail account all you need to do to click on the box that is atop the first email to select all. All the email is then checked. Then, just above the first email and in the center of the screen, click the link to select all xxx number of email. That will highlight every email in the label, (folder), so you can move or delete them at one time and not 50 or 100 at a time.
Once that is done, go to the Trash and repeat the process above to easily delete them from your account and free up space to continue receiving your mail.