One of the apparent advantages of Nabble as a platform is that it can be used either as a mailing list or forum. However, in practice this is a two edged sword.
When users subscribe to a topic, sub-forum or forum, they start to receive emails with the full text of any post and they can reply directly to that email and their reply will appear as a post on the forum.
NOTE: Taking a daily digests prevents replies by email.
This can produce a number of issues:
Nabble as a forum
Unless taking a digest subscription, the subscriber receives a copy of the original version of the post as it is made. If the original poster edits his message on the forum the subscriber will be replying to an earlier version of the post, creating the potential for untold confusion! You could say there's a good reason why conventional forum software only sends email notifications that a reply has been posted, as this forces users to visit the forum to reply and they will only see the current version of the original post and reply to that.
Most people these days top-post replies over quotes of the original message. This means that if there are a number of subscribers each posting email replies to posts in a topic, posts to the forum spawn countless copies of earlier replies. A few months down the line a search to find a particular post from amongst the masses of duplicates in the email tail of multiple repeated paragraphs becomes a nightmare.
Nabble as a Mailing List
If the majority of users respond by email and Nabble is just regarded as an archive of old posts that new subscribers can access to find the answer to FAQs, everything will be fine. However, if users start posting replies in the archive, in forum style, with no quote of message to which they are replying, particularly if they post very short replies or reply to messages more than a few days old it can be impossible to work out the context of what they have written. Such behaviour can be intensely annoying to those interacting with Nabble as a mail list.
Recommendation to Administrators
I recommend that Administrators be clear from the beginning about whether they are running a mail list or a forum and do their best to encourage users to adopt one form of interaction or the other as the two styles do not mix well.
If you have some elementary programming skills it is possible to work out how to modify your forum that could either encourage appropriate posting techniques or prevent inappropriate approaches.
For example it is possible to prevent "every topic" subscriptions to force people to take daily digests so they have to visit the forum to read the full text of a post or reply. More advanced users can change the code so email replies do not get posted to the forum and change the footer of emails to indicate this, so that subscribers are encouraged to visit the topic on the forum to reply.