I can't speak for Nabble but I think this is part of fundamental problem with Nabble.
Nabble tries to combine two technologies, mail lists and forums.
With a mail list you send an email and all you can do later is send a further email with a correction. When Nabble is used as a mail list, this works just fine.
On a forum you simply edit your earlier post. Good forum alerts readers when a post has been edited after a reply is posted. When Nabble is used as a forum this works just fine too.
The trouble comes when some users of a Nabble hosted "group" use the service as a mail list and some as a forum. The two are incompatable.
How would you ensure that those you are using Nabble as a mail list only get second emails and yet if someone who used to be a subscriber then switched to using the group as a forum has the earlier posts automatically convert to appear as an edited earlier post?
Just a Volunteer Nabble Helper - because the nice folk at Nabble have helped me! GregHelp - Building a set of answers to Nabble FAQs.