The mailbox holds one gigabyte per account.

Why there is a number at all

Disk is the one resource here that is neither elastic nor cheap. Forwarding costs bandwidth for a moment; storage costs money every month for as long as the mail exists, on a server paid for by donations.

A number that is the same for everybody is the honest version of that constraint. The alternative is an unstated limit that becomes a surprise.

When it fills

Incoming mail is refused with a permanent error, so senders get a bounce and know it did not arrive. Nothing is silently dropped, and nothing already stored is deleted to make room.

Staying under it

• Attachments are the whole story. A thousand ordinary messages weigh less than one photo album.
• Most clients can be told to remove mail from the server after downloading it, which turns this into a transit point rather than an archive.
• Aliases that receive automated volume are better left forwarding.

If you need more

Write and say what it is for. The number is a compromise, not a wall, and a real use is worth hearing about.