Thanks for your interest.
Decades ago when we first started with btinternet, email servers wanted to free space once mail had been downloaded. My understanding was POP worked that way. It was our responsibility to manage the mail.
Of course nowadays having your mail in the cloud is what most do, so they can use it on many devices especially now a phone. And that was IMAP. We are a mixture here.
Our most important user uses POP. In recent years the occasional login via webmail would show only email that had not yet been downloaded. I thought it was because it got deleted immediately it had been downloaded.
In the last few weeks the most important user has been having trouble. This has manifested in several ways and may well have roots here (although no settings changed.)
Performance has been erratic. Slow mail retrieval. The client inbox has been behaving strangely, with duplicate messages showing.
Difficulty sending email. Seems maybe OK early in the day, not good later in the day. Various messages, maybe being on a different ISP might be an issue (we do have a BT phone line but not broadband, as well as the paid for email.)
Re the inbox, as test I downloaded the emails early today, and then went to a different computer to login via webmail. I saw the emails that I had processed on the client. I returned to the client, asked check for new mail, and they downloaded again. It occurs to me there could be some timing issue particularly because of our different ISP, or POP now works on a sort of batch basis and the previous download hadn't completed correctly. No doubt other reasons.
The simplest thing seemed to be to ask about possible changes to POP3 behaviour from the server end. A particular change of interest would be what causes mail to be deleted from the inbox on the server.
