POP3, a widely used email protocol, can sometimes download the same emails repeatedly. This can be frustrating, but understanding the reason behind it can help you avoid it.

Unlike the newer IMAP protocol that syncs emails and folders between the email server and the email client, POP3 prioritizes downloading emails, typically from the Inbox folder, to your device for offline access. However, POP3 doesn't keep track of downloaded emails on the server itself. Instead, it relies on your email client (like Microsoft Outlook) to remember which emails have already been downloaded.

Here's where things can go awry:

• Memory Lapse: Your email client tracks downloaded emails using unique identifiers (IDs). If your email client forgets these UIDs due to software glitches, it might play it safe and download everything again.
• Incomplete Downloads: Interrupted downloads can also confuse your email client. Imagine downloading a file and the internet cuts out halfway through. Just like with files, a download hiccup can make your email client think it hasn't received everything, prompting a redownload.
• Inconsistent IDs: Sometimes, the order of emails on the server or changes within your email client can cause the UIDs to become inconsistent. This can lead your email client to treat all emails as “new” and download them all again.

Furthermore, as the number of emails increases in your Inbox, POP3 access may become inefficient. This inefficiency arises because the mail client requires more time to compare and verify the email list before initiating downloads. Consequently, the overall email access process gradually decelerates, resulting in slower performance over time.

To prevent this issue, it is advisable to opt for either one the following solutions:

If you require access to your emails from multiple devices or locations, it's advisable to utilize IMAP (Internet Message Access Protocol) instead of POP3. Unlike POP3, which is primarily designed for single-device access, IMAP offers seamless synchronization across multiple devices, ensuring that changes made on one device are reflected across all others.
See: How to 'switch' POP3 to IMAP in Microsoft Outlook

If you prefer to stick with POP3 access, we advise keeping the duration for which emails are stored on the server relatively short, such as less than 7 days. By doing so, you can enhance the efficiency of your daily email download sessions. This practice ensures that only recent emails are fetched during each session, minimizing the time and resources required for synchronization.
See: Ms Outlook (POP3): Configuring How Much Days Emails are Kept on Server

To prevent old emails from being redownloaded again, you can move them from the Inbox folder to another folder (e.g. Read folder) on the webmail.
See: How to move emails to other folders on webmail