If you are using POP3 to download emails and yet leaving them in the server, occasionally, you may experience old emails being downloaded to your mail client again.
As POP3 protocol doesn't keep records on which emails have been read or downloaded, the mail client (i.e. Ms Outlook) is the one that takes up the role - every time the mail client connects with the server, it will have to query the entire email list and ID, compare them to the records that it has already downloaded, and pick only those that has not been downloaded. These decision and calculations are all done at the mail client instead of the server. There were times when these list/IDs become inconsistent (server sort order, mail client checksum changed etc), and the mail client will treat the entire list as “new”, and could perform the download again.
Not only that, when the number of emails increases in your Inbox, POP3 access can get inefficient, as the mail client requires more time to compare and check the email list before downloading them, causing the overall email access process to become slower and slower.
To avoid such issues, we advise to:
If you need to share or access the mails from multiple devices, it is recommended to use IMAP to access your emails, as POP3 was not designed for multi-access.
If you would like to maintain with POP3 access, we recommend to keep the number of days to keep the emails on the server short (e.g. less than 7 days). This would at least increase the efficiency on daily email download session.