Freron MailMate

Many of us use MUAs in what you might call "one-pane mode". That is to say, messages always opened in separate windows, with no preview of them in a section of the same window as handles the mailbox. MailMate seems to be broken for this, which I'm afraid is a dealbreaker for its use for me.

It is possible to close the preview pane entirely, and to set doubleclick and return to open messages in new windows. However, marking messages as unread doesn't work properly. The application doesn't understand that "displaying" a message in a zero-pixel pane should not mark it as read. This means that just navigating through my mailbox selecting which message to display spuriously marks them all as read. (Or I can turn off the "Mark message as read after..." function, which just causes them all to never be marked as read, which is nearly as bad.)

While I recognize that this use is rare these days now that so many people have been infected by Outlook's multipane style, MailMate is close enough to working that you might consider it worth the engineering time to get the rest of the way there.

The desired behaviour can be seen in Mail.app: entirely closing the preview pane causes the application to not consider displaying messages there to constitute reading them.