When Outlook warns that your mailbox is almost full, it is not a disaster yet – but if nothing is done, you will soon be unable to send mail and eventually to receive it. Senders then get an error and you never learn the message did not arrive. Fortunately, space can usually be freed quickly.
How to check the mailbox size
- Outlook desktop app (classic): File → Info – under Mailbox Settings a bar shows the free space. For a breakdown by folder, use File → Tools → Mailbox Cleanup → View Mailbox Size.
- Outlook on the web and new Outlook: gear (Settings) → Account → Storage. It shows how much each folder takes and lets you empty folders directly.
In Microsoft 365 Business plans the primary mailbox is 50 GB.
Quick clean-up
- Large attachments. File → Tools → Mailbox Cleanup → Find items larger than – enter, for example, 5000 KB. Large messages are usually old photos, PDFs and presentations. Save the attachments you need to OneDrive or SharePoint and delete the message.
- Deleted Items. Deleted messages take up space until this folder is emptied. Right-click the folder → Empty Folder.
- Sent Items. Sent messages with attachments are often the largest folder. Sort by size and delete old large messages.
- Search by size. In any folder you can sort by size (Arrange by → Size) and find the largest messages in seconds.
- Newsletters and notifications. Old newsletters, system notifications and copies can be deleted in bulk by sender.
After deleting, it can take a few hours for Outlook to show the updated size.
Archive mailbox (Online Archive)
Microsoft 365 has a server-side archive mailbox that appears in Outlook as a separate folder and is searchable like the normal mailbox. Its size depends on the licence: with Business Basic and Standard the archive has a fixed limit, while Business Premium offers a much larger archive that expands automatically when needed.
An administrator enables it: Exchange admin center → Recipients → Mailboxes → user → Others → Manage mailbox archive. You can then drag messages to the archive or let it happen automatically.
A retention policy moves older messages to the archive automatically, for example everything older than two years. That way users do not have to clean up themselves. Retention policies can also work the other way – preventing certain messages from being deleted before a set time. They are configured in the Microsoft Purview portal, and it makes sense to set them consistently for the whole company.
Why not PST files
The old advice "export your mail to a PST file" creates more problems than it solves:
- the PST sits on one computer's disk and is lost with the computer unless it is backed up;
- large PST files corrupt easily;
- they are not visible from the phone or the web, and colleagues cannot access them;
- from a data protection point of view it is hard to say where and for how long personal data is kept.
Use the archive mailbox instead of PST.
Shared mailboxes and licence upgrades
A shared mailbox without a licence can hold up to 50 GB. When it fills up, archive old mail or assign the mailbox a licence. If a user's mailbox keeps filling up despite clean-ups, enable the archive or consider a licence with a larger archive, such as Business Premium.
When to call a professional
If the mailbox is already so full that mail has stopped flowing, or you need the archive and retention policies set up for several users, it is worth getting a specialist. ITABIKS helps as part of Microsoft 365 administration; for urgent issues call +372 53 888 229.