There are many FAQs asked about posts on a Nabble forum that are "not accepted by the mailing list". Invariably, the questions arise when someone posts to a Nabble archive of a mail list hosted elsewhere.
Most mail lists require that you are a subscriber before you can post to them. Registering on a Nabble forum does NOT subscribe you to the mail list. To do that you will need to email the appropriate subscribe address for that list or visit the administrative web page where you can subscribe.
When someone creates an archive for a mail list they simply subscribe Nabble to that list. (Note that anyone can do this. The list owner may know nothing of the existence of the Nabble archive.) Nabble then generates a forum that to which all emails received via that subscription are posted.
Those who register at the forum may then post to the archive. Nabble forwards any post made on the forum to the mail list, spoofing the "From:" address so that it appears to come from the poster and not Nabble's own subscription address.
Typically, a mail list requires a subscription before it accepts a post for distribution to subscribers and sends a rejection email. Nabble detects the rejection and marks the post as "not accepted by the mailing list".
In the Description area at the head of the Nabble archive there is always a link. Depending on the mail list's hosting software this might take you to the administration page for that list or the home page of the site that maintains the mail list. If necessary you will need to navigate to administration page for the list and enter your email address to subscribe. Often that will require that you respond to an email to confirm that you are the owner of the address before the subscription is recognised.
If you expect to interact with the mail list only through the archive, then you may wish to set a command in the mail list software to suspend the receipt of emails. Most mail list software has such a setting, or will respond to email sent to a "suspend address".