What happens as a mailbox fills up
- Warning. You receive a message saying the mailbox is nearly full. Everything still works.
- Prohibit send. You can no longer send email until you free some space. Incoming mail still arrives.
- Prohibit send and receive. Incoming mail is rejected and the people emailing you get a non-delivery report. This is the stage to avoid — you lose mail you never knew was sent.
In Microsoft 365 business plans those thresholds are 98 GB, 99 GB and 100 GB; in Exchange Online Plan 1 they are 49 GB, 49.5 GB and 50 GB. On Exchange Server and hosted Exchange, the administrator sets them.
Free space quickly
- Empty Deleted Items and Junk Email. Deleted messages still count until the folder is emptied.
- Find the big items. In classic Outlook, File → Tools → Mailbox Cleanup shows folder sizes and finds items over a size you choose. Sorting a folder by size works too.
- Deal with large attachments. Save the attachments you need to a shared drive or SharePoint, then delete the messages.
- Check Sent Items. Years of sent attachments are a common hidden culprit.
- Check shared mailboxes you have added — they have their own limits, and in Microsoft 365 an unlicensed shared mailbox stops at 50 GB.
Archive properly — not into PST files
The old fix was to move mail into a PST file on the PC. Avoid it for business mail: PST files are easily corrupted, are rarely backed up, cannot be searched by anyone else, and disappear with the laptop. Use an archive mailbox instead — it lives on the server, is searchable from Outlook and the web, and keeps the main mailbox small. Retention policies can move older mail into it automatically.
Our answer on email archive vs backup explains why you need both, and our guide to how long UK businesses should keep emails helps you decide what can go.
“The Outlook data file has reached its maximum size”
This message is about the cached copy of your mailbox on your PC (the OST file), not the mailbox on the server. Classic Outlook limits that file to 50 GB by default. Options: download less — set cached mode to keep, for example, only the last year of mail on the PC, with older mail still available online — or have an administrator raise the limit.
When you genuinely need more space
- Microsoft 365: Business Premium or an Exchange Online Archiving add-on gives an archive that grows to 1.5 TB.
- Hosted Exchange with us: mailboxes start at 50 GB and extend in 50 GB blocks at £2.50 + VAT a month, with no ceiling, and archive mailboxes are £3.00 + VAT per 100 GB. See our price list.