Have you existed thunderbird since the message arrived?

Is expunge set to occur on exit? (right click the account in the folder pane, select settings then server settings)

Matt, Do you mean exited (not existed) in your first question? If so, I had not exited, but I've done so now.

Also, expunge was not set to occur on exit, but I've now set it to occur.

I'm not sure that the above steps have resolved the issue. I still have e-mails showing in my browser client. I just manually moved a bunch of e-mails from Best Buy to a local folder, but when I sign into my account in the web client (Office 365/Outlook), the Best Buy e-mails that I moved still appear in the InBox.

outlook is rubbish, slow to respond and even slower to update. Does it move after an hour? it does sometime take that Microsoft subsidiary that long to move my pop downloaded mails to the pop folder in deleted.

The expunge is really "delete on server" IMAP is a funny beast and it has taken me years to realize I only know enough to get into trouble with it.

When you "delete" on IMAP you mark a message as deleted. What it then does is up to the server. In some implementations it simply leaves the messages right where they are and draws a line through them on the screen.

Expunge is when the real cleanup occurs.

I know this is not all that helpful. But what can I say. Outlook.com is rubbish unless you use a browser. Which is of course exactly what they want. There are no adds in Thunderbird. So no money.

Oh and just so you do not think it is just me. This is the next email I opened in my support list. https://support.mozilla.org/en-US/questions/1114967#answer-904540

Hi Matt. Thanks for your replies. Now that you've explained that IMAP is just marking the messages as deleted on the server side, I guess it just means that there is some delay on Microsoft's side before they actually take the action of deleting the message.

For reasons I'm sure you care nothing about, I selected Microsoft's Office 365 service as my e-mail hosting service when I got my own domain. I'm paying for it, and, consequently, there are no ads. "Free" Outlook may be a piece of rubbish, but I wouldn't know.

Thanks again.

I have the same problem with the messages not being deleted on the ISP. The problem is Thunderbird eventually stops receiving emails until I delete the messages flagged for expunge. I have an Android tablet. Same account on the tablet deletes the emails from the ISP when I tell the email client to delete. Why can't Thunderbird do the same thing?