Read emails not visible with multiple inboxes(combined inbox)

It depends on your expectations. I understand this list as the "all unread"-view, not "all inboxes". It makes sense to have unread mails in one prominent place, while read mails are accessible in the respective inboxes. I understand your confusion if your expectations are different, but after having used my Jolla for 10 months now (roughly), I found this concept is very good to work with.

(OTOH, I use my mailboxes for very different purposes, so for me it's always obvious where to look for a message that's no longer in the 'all unread' view, so your last sentence doesn't apply to me.)

Well, partly the issue is that this works differently from everything else, which is not necessarily a problem in itself if there is substantial benefit to this behaviour. That, at least, is debatable. The real problem is that this differs completely from the 'one account' setup where returning from an email goes back to the view you were in, with that email marked read. So by adding a new account, the UI behaviour changes.

Then, as mentioned, it is very tricky to then figure out where the email actually is if one wants to then mark it unread, to delete, or to deal with it in any other way. This is made further difficult by the fact that emails cannot be marked unread from within the email view itself, only from the list.

This is now making email all but useless for me.

I never had a "one account" setup, so that's another of your problems I cannot judge. (If this is the problem, the easiest solution would be to change the behaviour in the one account setup so it's the same as in normal use.)

Overall, it's a matter of taste, obviously, as I find the current behaviour very pragmatic. I find it hard to progress and find better UI solutions when the discussion includes words like "useless" for such an issue. After all, it's a UI decision that has its pros and cons just like any other. You lived with this aspect of the UI not even for a day; why not give the mail client a week or two to convince you?

It is useless ... for me, as I frequently mark emails unread if I need to deal with them later, but want to check the content. In any case you are making an assumption that I have only tried this for a day. I have been using it this way for a week now. I now frequently find myself going to my iPhone for mobile emailing as it does not demonstrate this behaviour.

This particular behaviour is Jolla-specific so there should be strong justification for it to work this way. I am not sure there is. In any case a more natural fit would be to have tabs or ways to see 'unread email' and 'all inbox emails', which would then work in both cases, both for single mailbox and multi mailbox usecases, if one really wants to see only unread email.

And yes, in the one account setup it works the same way as everywhere else.

I made the assumption because you wrote twoday that you added the 2nd account just now, which triggered your problem. Sorry if this was wrong.

So your proposal is to add a new "all inboxes" view to the UI. I guess that switch (all vs unread) could be implemented from the pulley menu.