Apart from those pesky mailing lists, all seems to be under control now.

I did notice something new though. Two emails (1 yesterday and 1 today) stay in received after I drop them on a label, even though all other similar emails are invisible in Received (the label is set to "hide messages from other views"). Any guesses?

I'd check the database to see if that fixes anything. There could be some corruption.

To rule out more corruption, you could also recreate all your labels (you can add duplicates with the same name). Then, drag the messages from the old copy of the label to the new copy of the label. Then, delete the old copy of the label. Do that for each label.

For the mailing list entry, you can delete the mailing list. The messages will still remain. The mailing list will be recreated next time a message for it comes in.

If you want all of those messages in the labels to show in Received, check "show hidden" for Received.

I did notice something new though. Two emails (1 yesterday and 1 today) stay in received after I drop them on a label, even though all other similar emails are invisible in Received (the label is set to "hide messages from other views"). Any guesses?

As these are not my mailing lists but someone else's, how can I make them appear as unread mails?

I think all you can do is check "mailing lists" under the "show" settings for the Unread message list.

Not sure why unread messages in the mailing list view for that list are not shown in bold though. There should be an unread count next to that mailing list also.

As these are not my mailing lists but someone else's, how can I make them appear as unread mails?

I do not see a button labelled "settings for this view"

When you left-click on "All Messages/Received" to open its message list, there will be a toolbar above the message list. On that toolbar will be a button. That button is a vertical (not slanted) wrench with horizontal lines on the left of the wrench and a drop-down arrow on the right of that wrench. You need to left-click it.

If you still don't see the button, right-click on that toolbar, goto customize and reset the toolbar to its defaults.

I do not automatically filter messages into labels and anyway, no labels show unread messages - only the Unread label and Inbox do. However, labels do have "hide from other views" ticked.

If setting "show hidden" and such for "All Messages/Received" doesn't help, there may be some other issue like corruption.

I do not see a button labelled "settings for this view". I've tried right-clicking Received but there seems to be nothing useful there either. "Properties"is greyed out at all times. I've tried clicking all the settings buttons, but nothing seems relevant. I have not seen a "show hidden" setting anywhere.

I do not automatically filter messages into labels and anyway, no labels show unread messages - only the Unread label and Inbox do. However, labels do have "hide from other views" ticked.