Some website owners allow emails to accumulate on the server. Once the disk quota of the hosting plan is filled, further emails will be “bounced” (returned to sender).
Sometimes website owners are unaware that emails are being stored on the server. To correct this, search Google for “leave messages on server” and follow instructions for your email program.
Mobile phones are often the cause of emails left on the server. If you use your phone for email, then be aware that your email account on your phone defaults to the IMAP protocol which does not store emails on your phone, but on the server. You then need to have a strategy to either remove them from the server or pay for sufficient space.
Your disk space can also get filled by uploading large images. This can also cause the “mail box full” bounce message for emails.
Test content takes up very little disk space.
Although WordPress optimises images that you upload, it leaves your original image on the server. e.g If you load a 3 Mb image from your camera, WordPress will fit it on your web page at say 100 Kb, but it will leave the 3 Mb image on the server.
So if you attempt to load 100 images @ 3 Mb each making 300 Mb total, your hosting plan quota will need to be high enough or it will fill up and only some of the images will actually be accepted.