New mail notifications not shown clearly enough (unlike Thunderbird)

There are 2 aspects to this problem:
1) A red "new messages" number is not always shown on the Dock icon if new mail had come in since Postbox was last foregrounded
2) Thunderbird highlighted in blue folders containing new messages that have come in since those folders were last viewed. Postbox only highlights folders with unread messages in bold. But this alone is not enough, since unread messages may be old.