If your an Aussie, you will already be aware of the fact user support for the three of the four big providers is rubbish. Which provider? My guess is Telstra and their dodgy outlook link up. But I might be wrong so please tell me.

Right click the account in the older pane, select settings from the menu.

In server settings change the setting to Delete immediately on download. Note if you are involved with the dodgy outlook/hotmail linkup. They don't delete mail when they get the command, they move it to a folder called POP and keep it. Having said that, it should really only ever reappear if you have set up an IMAP account . But if they did some weird migration after they dropped the contract, they might have moved the mail back to the inbox and marked it as read or something equally bizarre.

This ISP is a community group which inherited my 20+ year old account from my original ISP who kept losing his clients over time to Telstra and others and had to give up after 20 years.

In my Thunderbird POP server settings there isn't, nor has ever been a setting "Delete immediately on download".

Nor do I have the necessity to delete my emails in Thunderbird - that is the function, and reason I have used Mailwasher Pro since it was invented. I quickly preview my email there, deleting the unnecessaries.

Thunderbird downloads the mail I decide to keep and automatically sorts them into folders.

Several issues here. But the most obvious to me is you have not been removing the mail from the server in the past. Perhaps someone at the ISP did it for you when your account filled up.

You have the option in Thunderbird and you should be using it. with or without mail washer pro. If you do not you are doing the equivalent of asking the lady at the post off ice to photocopy all your mail and then give you the original and file the copy somewhere just in case you might need it some time in the future. Somewhat pointless and the cause of your current issue (the reapearance of your old mail) .

So right click the folder pane and make the necessary adjustments to your account settings.

You have got me started on how useless mail washer pro is. I have chosen not to go there until now, but it is a completely pointless addition to the process. Thunderbird has a spam engine that will do at least as well without the messing about you are currently doing now. And why? It was sold to us once upon a time as a product that prevented bad email getting onto our computers. At a time when everyone was using outlook express which happily executed any of script in the mail to infect you. So stopping them at the border stopped a huge issue created by defective mail software. The whole basis of their sales pitch is based on ignorance and fear. I would not give them a single dollar, let alone 30 of them, and you get to read your mail twice. How convenient.

So what if the email you receive has the worst virus in 30 years. Thunderbird does not execute scripts, so it is completely inert. There might be an issue with an attachment, but does mailwasher open all your attachments and scan them. No, that is why you have an anti virus, to detect threats in files you receive.

Does the sender get notified when you read their mail? Not if you do not display remote images (Thunderbird default). It is in the disclaimer information on just about every mail program usage report on the web, because consumer tracking uses images.

What about phishing... yep Thunderbird checks for that as well. It get lots of false positives because email marketers and phishers are very closely related in what they want to do. Mislead you while gaining the upper hand in information about you and your purchases.

Mailwasher was a useful product around 20 years ago. But it has not aged well.

"But the most obvious to me is you have not been removing the mail from the server in the past. Perhaps someone at the ISP did it for you when your account filled up"

Ever since 1998? Going back to the years 2000 to 2012 when my emails were 5,000 per week, now 500 to 1,000 a week - and my ISP [a friend] never told me? I don't think so. This has only happened in the last 18 months.

Mailwasher Pro has served me well for decades - it knows friends, it knows friendly domains, I've never had a problem in all that time.

If you send me an email with an attachment - I will trash it. I only accept expected attachments.

"So right click the folder pane and make the necessary adjustments to your account settings."

In Thunderbird I go to Tools/Account Settings and there is NO such option as has been described, and there never has been one.

I'm not some newcomer. I turn 77 in a few days, I have been involved in electronics for the last 63 years. I teach electronics and radio on the internet. I built my first microcomputer in 1976 with an RCA CPU.

I have never purchased an over the counter computer - all of my desktops have been custom built by myself. Going back to the 1980's I began programming in assembly language primarily for Motorola Microcontrollers.

I am also a Microsoft Insider. My Desktop has 11 hard drives totalling 27 Terabytes, 32 Gb RAM. I use up with my regular ISP around 135 Gb of data a month. No I don't watch/download films.

We'll see what the ISP says tomorrow.

Does the sender get notified when you read their mail?

NO.