Sometimes these issues don't relate to Kunena. I have had issues in the past where something similar happened and we tried everything we could in both Kunena and Jomsocial but email notifications never materialised. On two hosts SG and Inmotion we had this issue, but when we moved to Rochen they suddenly started to run. The problem was that the backlog was sent out so an apology had to be given to all recipients. The only difference I could see was that Rochen ran Cloudlinux and Litespeed as the web server where the others didn't, but it ids more likely to do with the configuration by the hosting company.
Also, though unlikely in this case, if your DMARC for email is either not set up, or not set up correctly the emails may be dropped by either the relay or during transit or by the recipients end. f at the relay end, that is whoever is hosting your email that may mean that is does not appear in the sent box or draft. The likes of Google, Yahoo, Paypal, etc all do this. This can also affect any payments via the paypal and other gateways as they require the DMARC to be set up properly
