New emails appear in a preview box in bottom right of screen

There is no preview of messages in Thunderbird. If you're using the message pane, and you see the message contents, that means the message has been opened. Hence it is also in your Inbox, if that is the folder you have selected.

but only about 1 in 20 is copied to the Inbox

I don't understand what that means. Please explain this in more detail. A screenshot illustrating the problem may help. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

Hi, this is Mike, Gislint's partner taking over the thread. I'm not a Thunderbird user myself, but hopefully I can describe the problem more succinctly.

First a correction: Gislint used the term 'preview box'. In fact, she was referring to a pop up box in the bottom right of the screen, which merely alerts that you've got new mail and shows the subject and sender. If she then opens her In-box these emails are often not there. If she then logs into her email account with the ISP, the new emails are on the server, but there seems to be nothing we can do to download them into Thunderbird. She received 11 new emails dated yesterday on the ISP server, and I listed them by time-received and cross checked against the T'bird Inbox. Only 8 had downloaded. From the server, I forwarded the missing ones to her email address and they were again received on the server. T'bird downloaded them into her Sent folder, but again they failed to reach her T'bird Inbox.

Another aspect of the problem (or maybe a different one), is that T'bird shows the number of unread emails with a number in bold alongside the Inbox. Yesterday evening, the number was 10, but there were only 9 emails highlighted in bold when the Inbox was opened.

I hope this is clearer now. If you wish to contact directly, my email address is [email]@waitrose.com Many thanks for any help you can give.

If she then opens her In-box these emails are often not there.

What is the account type - POP or IMAP?

Are there any other devices accessing the account, e.g. like a cell phone or another instance of Thunderbird on a different computer? If so, what is the account type on those other devices?

The account type is IMAP. There are no other devices accessing the account and no Thunderbird on another computer.

Thanks, Mike

Try to rebuild the index file of Inbox.

Right-click Inbox - Properties - Repair Folder

Do any of the missing messages show up?

Thanks. There were only 5 new emails to day and they all downloaded into Thunderbird.

I have done the 'Repair folder' steps as suggested and will monitor over next few days to see if any emails fail to download. Hopefully, the problem is resolved.

Many thanks for help so far.