POP3 leave-on-server settings control how email messages are downloaded from a mail server to a local email application. POP3 is designed mainly for downloading messages to a single device, and it may remove messages from the server after transfer. These settings can cause messages to download repeatedly, disappear from the mailbox, or appear on only one device. Learn more about what POP3 is.
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How POP3 handles downloaded messages
When a POP3 email application connects, it checks the mail server for messages and downloads them to the device. The downloaded copy is stored locally in the email application. POP3 is different from a setup that keeps the mailbox available in the same state across several devices because its main purpose is to transfer messages to a local device. mybox describes POP3 as a protocol that retrieves messages from a mail server and stores them locally.
After a message is downloaded, the POP3 application uses its message-handling settings to decide what happens to the server copy. The main choices are to leave the message on the server, delete it after download, or keep it for a defined retention period.
What the leave-on-server setting does
Leave on server means that the email application keeps a copy of a downloaded message on the mail server. The message remains available for another POP3 application or device to download.
This setting is useful when more than one device needs access to the same incoming messages. For example, a message downloaded by a computer can remain on the server so that a second device can also retrieve it. Because each device downloads its own local copy, the devices do not automatically share the same read, deleted, or folder state.
Leaving messages on the server also means that the mailbox can continue to hold messages after they have been downloaded. A retention-period setting can limit how long those server copies remain available.
Why messages are removed after download
Delete after download tells the POP3 application to remove the server copy after the message has been transferred to the local device. The message should then remain in the local mailbox on that device, but it may no longer be available for another device to download.
This behaviour matches the traditional POP3 model, where messages are downloaded and stored locally, often with the server copy removed after transfer. See the mybox description of POP3 message retrieval.
If messages appear on only one device, check whether that device deletes messages from the server after downloading them. The second device may connect after the first device has already removed the messages. In that situation, the second device has no server copies left to retrieve.
How retention periods affect mailbox copies
A retention period controls how long downloaded messages are kept on the server when leave-on-server is enabled. When the period ends, the server copy is removed. The local copy already downloaded to a device is separate from that server copy.
A short retention period can make messages disappear from the server sooner than expected. A longer retention period keeps them available for other POP3 devices for longer. The setting does not create a shared mailbox view. It only controls how long the server copy remains available for download.
Why POP3 can download the same message more than once
Duplicate emails can appear when a POP3 application downloads a server copy again instead of recognising that the message was already downloaded. This is more likely when the message remains on the server for multiple connections or when several devices retrieve the same mailbox.
A duplicate-detection setting helps the email application recognise messages that it has already downloaded. When duplicate detection is active, the application can avoid creating another local copy of the same message. If duplicate detection is not active or does not recognise the message, the same server copy may appear again as a new local message.
Leave-on-server and duplicate detection control different parts of the process. Leave-on-server controls whether the server copy remains. Duplicate detection controls whether a downloaded message is treated as a new local message when it is seen again.
Recommended settings for one device
For a mailbox used on one POP3 device, use settings that keep the local mailbox as the main copy:
- Disable leave-on-server if messages should be removed from the server after download.
- Use delete after download when the local device should receive the messages and the server should not retain them.
- Use duplicate detection to reduce the chance of the same message being added more than once.
With this arrangement, messages are downloaded to the selected device and the server copy is removed after transfer. The result is consistent with POP3’s single-device design. POP3 is primarily designed for downloading email to a single device.
Recommended settings for several devices
For a mailbox used on more than one POP3 device, keep server copies available:
- Enable leave-on-server on each POP3 device that needs to receive the messages.
- Do not use immediate deletion after download on the first device that connects.
- Choose a retention period long enough for the other devices to connect and download the messages.
- Keep duplicate detection enabled on each device to reduce repeated local copies.
Each device will store its own local copy. A message can therefore appear on several devices, but actions taken on one local copy do not automatically update the other local copies.
What to check when messages are missing or duplicated
- Check whether any POP3 device is set to delete messages after download.
- Check whether leave-on-server is enabled when several devices need the same messages.
- Check the retention period if server copies disappear after some time.
- Check that duplicate detection is enabled on devices that repeatedly download the same messages.
- Review the settings on every device connected to the mailbox, not only the device where the symptom appears.
If new messages stop appearing in an email application, reviewing the client settings is an appropriate first check. mybox recommends verifying email client settings when messages stop reaching the client.