Quick answer
You free up mailbox space fastest by first finding messages with large attachments and deleting them, including copies in the Sent Items folder. Then empty the Deleted Items and Junk Email folders, and archive old emails or move them to a local PST file. If the account is on Exchange or Microsoft 365, also check how much space the server archive takes up.
How to recognize the problem
- Outlook shows a notification that the mailbox is full
- New messages do not arrive, and senders receive an error message
- Sending an email stops with an error about insufficient space
- The Deleted Items and Sent Items folders grow day by day
- Synchronization on the phone constantly reports an error
Most common causes
Large attachments in old messages
A few emails with PDFs, images, or archives can take up hundreds of megabytes. Those files are stored in both Sent Items and Inbox.
Folders are not emptied
Deleted Items and Junk Email remain on the server until you manually empty them, so the space stays occupied.
Archive and other folders on the server
For Exchange and Microsoft 365 accounts, the archive may have a separate quota, but messages in other folders still count toward the account quota. That is why you should check the archive too, even though you do not see it in the main mailbox.
Synchronizing the entire account on the phone
If the phone stores all messages and attachments locally, space is quickly used up on the device and synchronization may report an error. This does not affect the mailbox quota on the server itself.
The account quota is small
Some accounts have a limit of only a few gigabytes, so they fill up faster than you expect.
Step-by-step fix
- 1
Check how much space is left
In Outlook, open
File > Info > Tools > Mailbox Cleanupand check the mailbox size. In Outlook on the web and in Gmail, look at the storage usage notification or the account storage page. Note how much space is left so you know how much you need to free up. - 2
Sort messages by size
In Outlook, click
View > View Settings > Columnsand add theSizecolumn, then sort descending. In Gmail, typesize:10Min the search box to get all messages larger than 10 MB. This is the fastest way to see what is using the most space. - 3
Delete the largest attachments
Open the messages with the largest attachments and delete them if you no longer need them. If you need the content, save the attachment to disk or cloud before deleting. Keep in mind that a message is permanently removed only when you empty
Deleted Items. Do not forget theSent Itemsfolder, where the same large messages often sit. - 4
Empty Deleted Items and Junk Email
Before emptying, check that there are no important messages in those folders, because emptying is irreversible. Right-click
Deleted Itemsand selectEmpty Folder. Do the same with theJunk Emailfolder. In Gmail, openTrashand empty it, then do the same with theSpamfolder. - 5
Archive old messages
In Outlook, select old emails and use the
Archivebutton, or drag them to a local PST file that you opened viaFile > Open & Export > Open Outlook Data File. In Gmail, useArchive, but keep in mind that archived messages still count toward the quota. - 6
Check the archive on the server
For Microsoft 365 accounts, open
File > Info > Account Settings > Account Settings, then theData Filestab, and check the archive size. If it is large, delete old items if you do not need them or move them to a local PST file. - 7
Reduce synchronization on the phone
In the Outlook app on your phone, open the account settings and find the synchronization period option, for example
Mail Days to Sync, and limit it to 30 days. This reduces space on the phone but does not free up the server quota. - 8
Check the quota and delete unnecessary folders
In Outlook on the web, open
Settings > General > Storageand see what is taking up space. If you are sure you do not need a folder, first make an archive or copy of important messages, then delete it and finally emptyDeleted Items.
If nothing helps
If you have deleted everything you can and space is still not freed up, check whether the retention period for items in Deleted Items is too long or whether there is a rule that returns messages to the mailbox. For Exchange accounts, the administrator can increase the quota or enable server-side archiving. As a temporary solution, move the oldest messages to a local PST file, which removes them from the server. Do not delete folders containing business correspondence without first archiving them, because that loses the communication trail.
How to prevent it
- Regularly empty `Deleted Items` and `Junk Email`, preferably once a week
- Send large files via a sharing link, not as an attachment
- Limit the synchronization period on your phone to 30 or 60 days to save device space
- Occasionally check the mailbox size in `File > Info > Tools > Mailbox Cleanup`
When to call a professional
If the account is a business account on an Exchange or Microsoft 365 server and the quota does not change even after deletion, contact IT support to check the rules, archive, and quota settings on the server.
IT infrastructure maintenance · Servers, networks, backup, security and helpdeskFAQ
Does deleting a message immediately free up space?
Not always. The message first goes to `Deleted Items` and stays there until you empty the folder. Only after emptying is space freed up on the server.
Does archiving in Gmail free up space?
No. Archived messages remain in the account and still count toward the quota. You free up space only by deleting them or moving them to a local file outside the account.
How much space do attachments in Sent Items take up?
Every sent email with an attachment keeps a copy of the attachment in the `Sent Items` folder. That is why deleting old sent messages with large attachments often frees up the most space.