If your ISP, or some intermediary in the delivery chain, is mistakenly classifying the mail as spam (and assuming the emails are not in your "spam" folder) this can be tricky , or close to impossible! I've had exactly this with some other "automated" senders. Originally, everyone's Inbox used to get clogged up with spam; then ISPs developed automated processes to divert it to a separate 'spam folder' (and you could retrieve stuff that they had put there mistakenly); but now some are just deleting it unilaterally - which is a real pain if it is something you actually want to receive.
You can try adding the forum email address to your address book / contacts, but in my experience this doesn't always work. (I.e. the address that notification emails are sent from. Not sure what this is - it might be helpful to mention it on the site somewhere, perhaps in a sticky in Technical & Account Support, as if we all added it to our contacts this might feed back to ISPs that it isn't a spam issuer).
It also helps if everyone on the site never classifies notification emails as spam, as that will certainly get back to ISPs and cause more problems. Change notification settings or just delete the emails if they are not wanted.
I don't know if the forum itself can do anything, with security certificates or similar, to identify the site as a "valid" non-spam sender.