When an IMAP mailbox reaches its storage limit, new messages may stop arriving or normal mailbox operations may be affected. If increasing the mailbox quota is not preferred, a practical option is to back up older emails to an Outlook Data File (.pst), verify the backup, and then remove selected old messages from the IMAP server to free up space.
This guide is intended for Classic Outlook for Windows, including Outlook 2016 and later supported desktop versions. Microsoft officially supports exporting Outlook mailbox data to a .pst file for backup purposes.
IMAP vs POP3: What Is the Difference?
IMAP (Internet Message Access Protocol) keeps the mailbox primarily on the mail server and synchronizes changes between supported devices and email clients. If you read, move, or delete a message in one synchronized client, the change can also appear on other connected devices.
This makes IMAP convenient for users who access the same mailbox from several devices. However, because emails remain on the server, they continue to use the mailbox’s available server storage.
POP3 (Post Office Protocol Version 3) works differently. It downloads messages from the mail server to the email client and focuses more on local storage. POP3 does not provide the same mailbox and folder synchronization model as IMAP.
Outlook can also be configured to leave a copy of downloaded POP3 messages on the server.
In simple terms:
- IMAP is better for synchronized, multi-device email access.
- POP3 is more suitable when email is mainly downloaded and managed on one computer.
Limitations of IMAP and POP3
IMAP Limitations
IMAP depends on server storage. Because emails remain on the mail server, the mailbox can eventually reach its storage quota, especially when it contains many emails and large attachments.
Deleting an email from an IMAP mailbox can also synchronize that deletion with the mail server and other connected devices.
An IMAP .ost file should also not be treated as an independent backup. Microsoft describes the .ost file used by IMAP as an offline synchronized copy of the information stored on the server.
POP3 Limitations
POP3 can reduce reliance on server storage when messages are downloaded locally and later removed from the server. However, it also has limitations.
Folders, read/unread status, and other mailbox changes are not synchronized in the same way as IMAP. Users who access their email from multiple devices may therefore see an inconsistent mailbox.
Because emails may be stored mainly on the computer, the local Outlook data should also be backed up. A computer or storage failure could otherwise result in data loss.
How to Check Whether Outlook Uses IMAP or POP3
To check the current account type in Classic Outlook:
- Open Outlook.
- Select File.
- Select Account Settings > Account Settings.
- Select the email account.
- Check the account Type.
Microsoft confirms that the Account Settings window displays the type of email account being used.
If the account is shown as IMAP, you can proceed with the backup procedure below.
Back Up the IMAP Mailbox to a PST File
Before exporting, make sure Outlook has synchronized all emails that need to be backed up.
Microsoft recommends checking Account Settings > Account Name and Sync Settings before exporting. Where the download range option is available, move the slider to All if you want all available email downloaded before export.
Wait until the required folders and emails are visible in Outlook before continuing.
To export the mailbox in Classic Outlook:
- Select File > Open & Export > Import/Export.
- Choose Export to a file, then select Next.
- Choose Outlook Data File (.pst), then select Next.
- Select the email account or folder that you want to back up.
- If exporting the full account, make sure Include subfolders is selected.
- Select Browse and choose a location and file name for the
.pstfile. - Select Finish.
- Add a password if required by your company’s policy.
These steps are documented by Microsoft and are also consistent with ServerFreak’s Outlook 2016 PST export guide, including the recommendation to select Include subfolders when exporting the account.
Microsoft confirms that exporting Outlook data to PST creates a copy. It does not automatically remove the original emails from Outlook or the server.
Important: Do not assume that completing the PST export has already freed mailbox storage.
Verify the Backup and Free Up Storage
Do not delete emails from the IMAP mailbox immediately after the export finishes.
First, open and inspect the PST backup in Outlook. Check:
- Inbox
- Sent Items
- Important custom folders
- Older emails
- Email attachments
- The expected date range
This helps confirm that important company emails have been successfully backed up before anything is removed from the server.
For important business data, it is also recommended to keep another copy of the completed backup in a company-approved backup location.
After the PST has been successfully verified, old emails that are no longer required online can be removed from the live IMAP mailbox.
You may also need to empty Deleted Items or Trash before the mail server reports the recovered storage.
Microsoft specifically notes that PST files can be used when an online mailbox is nearly full and can serve as archives or backups.
Important: Exporting to PST alone does not reduce server storage. The original server emails must be removed after the backup has been verified.
6. Optional: Switch from IMAP to POP3
Switching to POP3 is optional. It should only be considered if the email provider supports POP3 and the user understands the differences described earlier.
Before configuring POP3, obtain the correct:
- Incoming POP3 server
- Outgoing SMTP server
- Port numbers
- Encryption settings
- Username
- Password or authentication requirements
Instead of attempting to directly convert the existing IMAP account, configure the email account using the provider’s POP3 settings where supported.
Keep the existing IMAP configuration until the POP3 account has been successfully tested.
After configuring POP3:
- Send a test email.
- Confirm that incoming email downloads correctly.
- Check whether Leave a copy of messages on the server is enabled.
- Decide how long server copies should be retained.
- Ensure the local Outlook data is included in the company’s backup process.
Microsoft confirms that POP3 accounts can use the Leave a copy of messages on the server option.
If server copies are retained indefinitely, the mailbox can still become full again. Therefore, POP3 does not automatically solve a storage problem. Server storage is reduced only when emails are actually removed from the server.
Frequently Asked Questions (FAQ)
1. Does exporting email to a PST file immediately free IMAP storage?
No. Exporting creates a copy of the data. The original emails remain on the mail server until they are removed from the live mailbox.
2. Can I delete old emails as soon as the PST export finishes?
It is safer to verify the PST first. Check important folders, older emails, and attachments before deleting anything from the IMAP mailbox.
3. Is the IMAP .ost file already a backup?
No. Microsoft describes an .ost file as a synchronized offline copy of information stored on the server. It should not be relied on as an independent archive of emails that you intend to remove from the server.
4. Will POP3 prevent the mailbox from becoming full again?
Not necessarily. If POP3 is configured to leave all downloaded messages on the server indefinitely, server storage can continue to increase.
5. Should every IMAP user switch to POP3?
No. IMAP is generally more convenient for users who access the same mailbox from multiple devices. POP3 should only be considered when local email storage is suitable for the user’s workflow and the email provider supports it.
6. What is the safest order for this process?
The recommended order is:
Synchronize required emails → Export to PST → Verify the PST → Keep a backup copy → Remove selected old emails from the server → Confirm storage has been freed → Consider POP3 if required.
