The Received date shown by Thunderbird for messages in any folder, used to sort messages, etc. is the date when the message was placed in the maildrop by the last mailserver. In some situations, it would be nice to be able to see in the user interface the date and time when the message has been actually delivered from that maildrop to the user's local mailbox.
Some other mail user agents have this feature. While some of such MUAs, e.g. The Bat!, use a different mailbox format, not Unix mbox, it should be relatively easy to implement this feature in mbox-based Thunderbird: there is the date and time in the first From_ line for the message in mbox, so we just need to show it somehow in the UI.
Such idea was briefly mentioned on Bugzilla years ago: see https://bugzilla.mozilla.org/show_bug.cgi?id=190337#c6. I think the time has come to finally implement it.
I have posted a request for enhancement at https://bugzilla.mozilla.org/show_bug.cgi?id=1679262; please support it if you agree that this is a useful feature to have.
There are many use cases where one would need it. Suppose, for example, that there are 5 messages received by Gmail server yesterday, but I was not able to download them because of some connection/server-side spam filtering/whatever problems. Now I have finally downloaded them, as well as 20 other messages that arrived to Gmail server today. If I look at the message list in Thunderbird ordered by Received: date, I may not realize that there are not 20 but 25 new messages, i.e. those I received today, which need some action from me.