What to do when your mailbox is full

Nobody sets out to fill a mailbox. It happens quietly over years: a few large attachments a week, a Sent folder nobody has ever looked in, and a Deleted Items folder holding a decade of things that were never really deleted.

Then one morning the warnings start, and shortly after that the mailbox stops accepting new mail. That last part is what makes this worth dealing with before it becomes urgent, because a full mailbox does not just inconvenience you. It quietly turns away messages from clients, and you may not find out for days.

How much room do you actually have

Most business mailboxes on Microsoft 365 come with around 50GB, which sounds enormous until you consider that a single project with photographs or drawings attached can run to hundreds of megabytes.

It is worth knowing the ceiling applies to more than your inbox. Sent Items, Deleted Items, Junk Email and any folders you have made all count towards the same total. In practice, Sent Items is very often the biggest single folder in a full mailbox, because everything you have ever sent, with every attachment you have ever sent, is sitting in it.

Find out what is taking up the room

Do not start by deleting things at random, and do not start with the oldest messages. Start with the biggest, because a handful of them will usually recover more space than hundreds of ordinary emails.

Both Outlook and Outlook on the web let you sort a folder by size rather than by date. Sort Sent Items that way and you will usually find the culprits within seconds: a few enormous attachments, often sent to somebody who already has them.

Look in the places nobody thinks to look:

• Sent Items, which is often the largest folder of the lot
• Deleted Items, which holds things until it is actually emptied
• Junk Email, which quietly accumulates for years
• Any shared mailbox you have access to, which has its own separate limit

Clear what you genuinely do not need

Some things can go without much thought. Newsletters you have already read, delivery notifications, calendar invitations for meetings that happened years ago, and the endless “thanks” replies that end a conversation. None of that is a record of anything.

Large attachments deserve more care, and the useful question is not “do I still need this email” but “is this the only copy of this file”. If a drawing, contract or photograph exists in your shared drive or document system, the copy in your mailbox is duplication and can go. If your mailbox is the only place it lives, save it somewhere sensible first, then delete the message.

Two warnings worth heeding. First, emptying Deleted Items is genuinely permanent, so work through it rather than tipping it out without looking. Second, some records need keeping for legal or tax reasons, and a mailbox tidy-up is not the moment to make that call on the fly. If in doubt, archive rather than delete.

Archiving, done properly

Archiving means moving older mail out of your main mailbox while keeping it available. It is the right answer when you need to keep things but do not need them under your nose.

On Microsoft 365 the sensible route is an online archive, a second archive mailbox that sits alongside your main one with its own generous allowance. It appears in Outlook as an extra folder, it is still searchable, and it is still backed up and protected by the same security as the rest of your account. Mail can be moved into it manually, or a policy can move anything older than a chosen age automatically, which is the version most people prefer because it then looks after itself.

Depending on the plan you are on, an online archive is either already included or can be added to a mailbox.

What not to do

The tempting shortcut is to export years of email into a file on your own computer and delete it from the server. Outlook on Windows will happily do this, and it does free up space.

It is also how people lose a decade of email in one go. That file lives on one machine, so it is usually outside whatever backup protects the rest of your business. It is not searchable from your phone or laptop, it is prone to corruption as it grows, and when that computer is eventually replaced the file is often forgotten. We have been asked to recover more than one of these, and it does not always end well.

If email is worth keeping, it is worth keeping somewhere looked after.

When tidying up is not enough

Sometimes the honest answer is that the mailbox is the right size for the job and the allowance is not. If someone genuinely deals in large files all day, no amount of housekeeping will change the arithmetic.

At that point there are two sensible moves: add an online archive so the main mailbox only holds recent mail, or move that person to a plan with a larger allowance. Both are straightforward, and worth doing deliberately rather than as an emergency.

It is also worth asking whether email is the right home for the files in the first place. If large documents are routinely being sent back and forth internally, sharing a link from a document system instead of attaching a copy solves the storage problem and half a dozen version-control problems at the same time.

A quick word on shared mailboxes

Shared addresses such as info@ or accounts@ have their own limits, and because no single person feels responsible for them they fill up more often than personal mailboxes do. They are also the ones you can least afford to have bouncing mail, since they are usually the addresses on your website.

Worth putting a reminder in the calendar to check them once or twice a year.

If you would rather not spend an afternoon on it

Working out what is safe to remove, setting up an archive, or moving somebody to a different plan is quick when you have done it before and fiddly when you have not, particularly across a whole team.

Business email is part of what we look after for clients, so if a mailbox is filling up, or you would like archiving set up before it becomes a problem, get in touch and we will sort it out. Our guide to Microsoft 365 email on your phone and desktop covers the related question of getting mail set up properly in the first place.