The sixty-second answer
Size a mailbox from attachments, not message count. Text is negligible; files are everything, and email encoding inflates a binary file by about a third on the way through. Estimate a typical week of attachments, multiply by fifty-two, add the third, then keep at least six years for anything that is a business record.
Words are free. Files are not.
People estimate mailbox size by counting messages, and the estimate always comes out wrong, because the messages are not what fills the box.
A long email is a few kilobytes. You could write one every working hour for a decade and not trouble a modern quota. What consumes storage is what gets attached: photographs from a job site, a scanned bill of lading, a PDF quote, a drawing, a spreadsheet, a short video from a customer showing you the problem.
And attachments do not arrive at their original size. Email message bodies are text, so binary files are re-encoded to pass through safely. The MIME specification is precise about the cost: Base64-encoded data "are consistently only about 33 percent larger than the unencoded data" [1]. A 6 MB photo is roughly 8 MB inside a message. A 30 MB folder of site photos is around 40 MB by the time it lands.
Then it lands more than once. Send those photos to two colleagues on your own domain and you are storing the sender's copy plus two recipient copies. Reply-all with the attachment still quoted and you have done it again. Nothing about this is a bug; it is what email is. But it means storage grows as roughly attachments times recipients times 1.33, and that multiplier is why mailboxes surprise people.
How to actually estimate it
Do this from your own habits rather than from a table on a vendor's website.
Pick a normal week - not your quietest, not your year-end. Add up the size of the files you sent and the files you received. Do not count the message text. Multiply that weekly figure by 52 for an annual figure, add a third for encoding, and then multiply by the number of years you intend to keep mail in the mailbox.
Three shapes come out of this exercise, and most Canadian businesses fall clearly into one. This matters because the population is overwhelmingly small: as of December 2024 there were 1.10 million employer businesses in Canada, and 1.08 million of them - 98.2 per cent - were small businesses [6]. Enterprise sizing advice is written for a different animal.
Text-shaped businesses. Consultants, agents, professional services. Mail is mostly correspondence with the occasional PDF. Even a decade of history stays modest, and storage is not a factor in choosing a provider.
Document-shaped businesses. Bookkeepers, brokers, anyone exchanging signed paperwork. Steady, predictable growth: a few hundred megabytes a year, per person, and the curve is flat.
Image-shaped businesses. Trades, contractors, inspectors, real estate, anyone whose customers send photographs from a phone. This is where mailboxes fill, because a single site visit can produce more data than a month of correspondence. If this is you, size generously and check the number annually.
What happens when it fills
Nothing graceful. A full mailbox refuses incoming mail, and the sender receives a bounce that tells them your business could not accept their message - which reads to a customer as carelessness rather than as a technical limit.
Some of this is negotiated before transmission. SMTP has an extension for declaring message size, defined "whereby an SMTP client and server may interact to give the server an opportunity to decline to accept a message (perhaps temporarily) based on the client's estimate of the message size" [2]. That is why a large attachment sometimes fails instantly rather than after a long upload. It is a courtesy, not a rescue: the message still does not arrive.
The practical lesson is that quota should be monitored, not discovered. Any provider worth using will tell you the usage per mailbox; the number to watch is the trend, not the current figure.
Cleaning up properly
When a mailbox does get tight, most "cleanups" free nothing, because deletion in email has two stages. IMAP distinguishes marking a message for removal from removing it permanently, alongside the other operations it supports - creating and renaming folders, searching, and selectively fetching message attributes, texts and portions thereof [3]. Most mail clients layer a Trash folder on top of that, and Trash still counts against quota. So does Junk. So, on many setups, does Sent - which is where the large outgoing attachments live.
An effective cleanup, in order: search for the largest messages rather than the oldest; deal with Sent before Inbox; empty Trash and Junk; and only then look at archiving. Sorting by size finds in ten minutes what sorting by date will not find in an afternoon.
The other trick worth knowing is to stop the growth at the source - link to large files rather than attaching them, especially for anything you will send to several people at once.
The floor you cannot delete below
Before you plan a deletion policy, note that Canadian law sets a minimum you have to keep.
Section 230(1) of the Income Tax Act requires every person carrying on business to keep records and books of account. Subsection 230(4)(b) requires them to be retained "until the expiration of six years from the end of the last taxation year to which the records and books of account relate", and subsection 230(4.1) adds that a person who keeps those records electronically "shall retain them in an electronically readable format" for the same retention period [4]. Email that constitutes a record - a purchase order, an invoice, a confirmation of terms - is inside that rule. Six years is not a suggestion.
Pulling in the other direction is the privacy obligation. PIPEDA's Schedule 1 provides that information which is no longer required "should be destroyed, erased, or made anonymous", and that organizations "shall develop guidelines and implement procedures to govern the destruction" of personal information [5]. Keeping everything forever is not the safe default it appears to be.
The reconciliation is a written retention period: keep records for the statutory term, dispose of what is past it on a documented schedule, and do not let quota pressure make that decision for you at 4pm on a Friday. We work through this properly in business email retention and records in Canada and business email privacy obligations in Canada.
Storage is not backup
A large mailbox feels like an archive. It is not one. Everything in it is subject to a single deletion, and because every device syncs to the same store, a mistaken deletion on a phone reaches the laptop before you have put the phone down - see getting your business email on your phone and laptop for why that is the intended behaviour.
If the mail matters, it needs a copy somewhere the mailbox cannot reach. That is a separate exercise with a separate budget, covered in how to back up business email in Canada. Buying more quota does not do it.
How many mailboxes am I sizing?
One more thing that skews estimates: people size "the company's email" as a single pool when it is actually allocated per mailbox. If storage is per seat, the question is not how much your business needs but how much your heaviest user needs, because they will hit the limit while everyone else sits at a fraction of theirs. Count the seats first - how many email addresses your business actually needs - then size the busiest one.
Where we sit
MapleMail states storage per mailbox rather than describing it as unlimited, because unlimited storage is either untrue or priced as though it were not. The current allocation for each plan is on the pricing page, along with what happens if you need more.
Mail is held in Canada, on your own domain, with standard IMAP access - which also means you can copy the whole mailbox out with ordinary tools if you ever want to archive it yourself or move on. We would rather tell you the number and let you check it against the estimate you did above than sell you a word.
If you do that estimate and it comes out larger than you expected, that is not a reason to buy a bigger plan straight away. It is usually a reason to stop attaching things.