Change a Mailbox's Storage Size

Every mailbox either shares your account's storage with the other mailboxes or has a fixed size of its own. You can switch a mailbox between the two, or change its size, at any time. Nothing in the mailbox is moved or deleted when you do.

Shared storage or a fixed size

• Shared storage is how every mailbox starts. The mailbox uses whatever space your account has free, together with every other mailbox on shared storage. One busy mailbox can grow while quiet ones stay small.
• A fixed size reserves an amount for this mailbox alone, for its email and its own Drive files. Nobody else can use that space, and the mailbox stops at it.

A fixed size suits a mailbox that must never run short, such as a manager's inbox, or one you want to keep small, such as an inbox that collects automatic alerts. For everything else, shared storage is usually the better choice, because no space sits unused.

Change it in the dashboard

1. Open Mailboxes.
2. Open the three-dot menu on the mailbox's row and choose Edit storage. On the mailbox's own page, Edit storage is at the top.
3. To give the mailbox a fixed size, untick Shared storage, then type the size and pick MB or GB, or drag the slider. Available: shows the most you can give it right now.
4. Save.

To put a mailbox back on shared storage, tick Shared storage and save. Its reserved space goes back to everyone at once.

Mail apps that are already connected pick up the new size the next time they sign in. This works the same way for shared mailboxes, such as support@.

When a size is refused

A fixed size has to fit in the space that is still free. If it doesn't, a message under the field explains why, and the mailbox keeps the size it had.

• Your account doesn't have enough free space. The other fixed sizes and open invitations with a fixed size already take part of your storage. Choose a smaller size, make another mailbox smaller, move one back to shared storage, or add storage to your account. A team member limited to some domains, such as a client's admin, is asked to contact their administrator instead.
• The domain has a client storage limit. If you resell email and gave a client a storage limit, a fixed size in their domains must also fit what that limit has left. You see how much is left and that you can raise the limit in Team & clients. The client's own admins see a short message asking them to contact their administrator.
• The mailbox already holds more. In a domain with a client storage limit, a mailbox can't be made smaller than what it already stores, because that space would look free while the mailbox still fills it. Delete mail or files first, or choose a size at least as large.

Moving a mailbox to shared storage is never refused for lack of space.

Making a mailbox smaller

Outside a client storage limit you can give a mailbox a fixed size smaller than what it already holds. Nothing is deleted, but the mailbox is full straight away: new mail bounces and Drive uploads are refused until it is back under its size. Reading and deleting keep working, so the person can tidy up. Check how much the mailbox holds first: the Mailboxes list shows its email, and its Limits tab shows its Drive files.

From the API or an AI agent

The same change is available to scripts and AI agents, with the same checks:

• REST: PATCH /api/v1/mailboxes/{id}/storage with storage_allocation_mb set to a size in MB, or to null for shared storage. It needs the mailboxes:write scope and an Idempotency-Key header.
• MCP: the set_mailbox_storage tool.

A refusal comes back with a code you can act on: storage_pool_exceeded when your account has too little free space, client_storage_limit_exceeded or client_storage_below_usage in a domain with a client storage limit, and mailbox_unavailable for a mailbox in Recently deleted. See the REST API overview for the details.

New mailboxes can get a fixed size from the start, in the dashboard, in bulk and through the API. See Create a TrekMail mailbox.

Common questions

Does changing the size move or delete anything? No. Mail and files stay where they are. Only the amount the mailbox may use changes.

Who can change it? The account owner, and team members whose role can change mailboxes in that domain, such as Client or Mailbox Operator. See Team roles and what each one can reach.

Does a fixed size cover Drive too? Yes. A mailbox with a fixed size keeps its email and its own Drive files inside that size. On shared storage, both use your account's free space.

Can I change a mailbox in Recently deleted? No. Restore it first, then change its size.

Related

• Mailbox storage and pooled quotas explained
• Give each client their own storage limit
• Limiting or turning off Drive for a mailbox