Mailbox Storage Quotas: Stopping One Archive Starving the Rest

Pooled storage is the arrangement that makes large numbers of mailboxes affordable, and it has one failure mode. The pool is shared, so a single mailbox with an enthusiastic attachment habit can consume what forty colleagues were relying on. Mailbox storage quotas are how you prevent that, and they're the setting most people configure after the problem rather than before.

This page covers what the pool actually is on each plan, how to size a quota sensibly, and what happens when one is reached.

The Pool You're Dividing Up

Storage is allocated to the account rather than per mailbox, and the totals are:

The gap between those two columns is the thing to notice. Agency permits a million mailboxes and provides 200 GB, which is 200 KB each if you actually created a million. Nobody does, but the arithmetic makes the point: the mailbox count is generous and the storage is the real constraint, so mailbox storage quotas are what keep the two in a sensible relationship.

For a realiztic case — five hundred mailboxes on Agency — the pool divides to roughly 410 MB each. That's comfortable for correspondence and tight for anything carrying photographs or drawings.

Sizing a Quota Without Guessing

The temptation when setting mailbox storage quotas is to divide the pool by the mailbox count and use that number. It's a reasonable starting point and usually wrong, because mailboxes aren't equal.

A better approach is three tiers. Give the small majority — addresses that exist to receive occasional mail — a modest quota, because they'll never approach it. Give ordinary working mailboxes a middle figure. Leave a genuinely heavy handful either unrestricted or generously provisioned, and watch those.

The reason this works is that mail volume is not evenly distributed. In most organizations a small number of mailboxes account for most of the storage, and mailbox storage quotas sized for the average punish everyone while barely affecting the ones causing the problem.

Leave headroom in the pool too. If quotas sum to exactly the pool, you have no room for a mailbox that needs raising urgently, and raising one means lowering another at the worst possible moment.

Hitting the Limit

A mailbox reaching one of your mailbox storage quotas stops accepting new mail, and the sender receives a bounce saying so — the over-quota response defined in RFC 3463. That's the behavior you want, uncomfortable as it sounds — the alternative is silently discarding mail, which is worse in every way.

The practical consequence is that quotas need warning before they need enforcement. Somebody who discovers their limit through a customer telling them their message bounced has been failed by the arrangement, not helped by it.

So the useful discipline is checking usage periodically rather than waiting. The mailboxes approaching their limits are visible before they arrive, and a five-minute review each month prevents the entire category of incident.

When the Pool Itself Runs Out

Mailbox storage quotas manage distribution, not total capacity. When the pool itself is exhausted there are three options, in ascending order of cost.

Clear what shouldn't be there. Most full mailboxes are full of attachments, and most of those attachments exist in several places at once — in the mailbox, in Sent, and again in every forwarded copy. Moving to share links rather than attachments removes the duplication at source, which is covered in the attachment size limit.

Add the Drive add-on. Storage from $3.20 a month, which is almost always cheaper than moving up a plan tier when storage is the only thing you're short of. This is the right answer when the mailbox count and feature set are fine and only the pool is tight.

Move up a plan. Correct when you need the other things a higher tier brings — more filters, more shared mailboxes, more external accounts — and the storage increase is a bonus rather than the point.

Doing them in that order is usually cheapest, because the first one is free and frequently sufficient.

Quotas and Archives

The specific case that prompts most people to set mailbox storage quotas is an archive: one long-serving mailbox holding a decade of correspondence, quietly occupying most of the pool.

That mailbox is not misbehaving. It's doing exactly what it should, and the correspondence in it is probably the most valuable mail the business holds. Capping it is the wrong instinct.

The better arrangement is moving the old material out and leaving the working mailbox lean. An IMAP export produces a copy you control, which can live in storage where it costs a fraction as much per gigabyte and remains searchable when needed. The mailbox then continues with a normal quota, and the history isn't lost or squeezed.

Retention policy belongs in the same conversation. If nobody has decided how long mail is kept, the answer defaults to forever, and forever eventually collides with any pool of any size.

What Actually Counts Toward a Quota

Everything stored in the mailbox, which is broader than people assume.

Sent mail counts, and on a mailbox that sends attachments it is frequently the largest folder. Trash counts until it is emptied, so a user who deletes a year of mail and never empties the folder has freed nothing. Drafts count, including the abandoned ones nobody remembers writing.

That order is also the order to check when a mailbox is unexpectedly full: Sent first, Trash second, and only then the inbox everybody assumes is the problem.

Setting Them Before You Need Them

The recurring theme is that mailbox storage quotas are cheap to configure in advance and awkward to introduce during a shortage.

Applied at provisioning time, a quota is invisible — nobody notices a limit they never approach. Applied to a mailbox already over it, the same limit is an incident, because mail stops arriving for somebody who has done nothing wrong.

So the advice is simply to set them when creating mailboxes rather than when the pool tightens, particularly if you're provisioning in bulk, where the quota can be part of the same operation as the creation. That's the point at which it costs nothing at all, described in creating mailboxes in bulk.