Why is Gmail full when I barely have any email?

Gmail shares its space with Google Photos and Google Drive, and in most cases the photos are what fills it. Go to one.google.com/storage to see the split between mail, photos and files, and you will know where clearing actually pays off.

Is space freed as soon as I delete something?

Not until the bin is emptied. Deleted mail sits there for thirty days and takes up space the whole time. Empty the bin, and the spam folder too, and the space shows up within a few minutes.

What happens to email sent while my mailbox is full?

The sender gets a message saying it could not be delivered, and it will not arrive on its own once you have cleared space. Ask whoever wrote to you to send it again after you have made room.

Your mailbox is full and new email is bouncing

When a mailbox is full, new email is refused and whoever wrote to you gets a bounce message instead of a delivery. It takes about fifteen minutes to sort out. If you use Gmail, the key thing to know is that the space is shared between Gmail, Google Photos and Google Drive, which explains how it fills up without you receiving many messages.

First, see where the space went

If you use Gmail: go to one.google.com/storage in a browser. It shows how much of your space is mail, how much is photos and how much is files.
If you use Outlook: click the gear icon at the top right, choose All settings, then General, then Storage.

Look at which of the three is largest and start clearing there. If photos are what fills the space, going through your email will not achieve much.

Save what you might need before deleting

Email with receipts, contracts, warranties and certificates is hard to get back once it is gone. Open those messages, choose Print and then Save as PDF, and put the file in a folder on your computer. Do this before you empty the bin, because anything emptied from there cannot be recovered.

Try this first

Start with large attachments. In Gmail, type has:attachment larger:10M in the search box and press Enter to bring up the heaviest messages. In Outlook, search and then sort the results by Size.
Empty the spam folder. In Gmail, open Spam in the left column and choose Delete all spam messages now.
Empty the bin. Only then is the space actually freed. In Gmail it empties itself after thirty days, but you can do it straight away.
Clear old newsletters. In Gmail, type category:promotions older_than:1y in the search box, select everything and delete.
Unsubscribe from newsletters you do not read. In Gmail the Unsubscribe link sits at the top of the message itself, next to the sender's name.
Reload the page and check the storage figure again. It updates with a few minutes of delay, so wait a moment before drawing any conclusion.

If that did not help

If you use Google Photos, that is usually where the space has gone. Go to photos.google.com, open the Bin and empty it, then go through long videos, which weigh more than anything else.
Go to drive.google.com, sort by storage used, and delete large files you do not need. Empty the bin there too, because it is a separate one.
If you would rather not clear anything, you can buy more space. Google One costs a small amount per month for a hundred gigabytes, and Microsoft 365 gives you more space in Outlook and OneDrive. That is a reasonable choice if you have years of mail you want to keep.

If the mailbox fills up again soon after clearing, if you are unsure what is safe to delete, or if you would rather move old mail to your computer than throw it away, ask us in the chat and we will go through it together.