Suggest you also contact your email provider's technical support and ask about external spam filtering.
They will need some detailed information, so plan on a 30 minute phone call excluding hold time.
Why?
Any entity with large fiber backbone connection to web has 2 battle fronts.
One is wild wild west of open internet communications, or outside interface.
Second is the FILTERED inside interface.
No one can tolerate the entire unfiltered web inside their business as the amount of data is staggering. Some even use layers of filters.
To make the filtering easier, rules are often IP based, or domain based. There are also machine learning tools that use statistics to evaluate if, say a site is sending emails with exactly 5 advertising links in each and labeling it unwanted or spam. Simplest tools use counts. meaning any domain sending more than XXX emails a day to my server, has to be spam - right?
These are supposed to be put in your spam folder, but not if they are labeled as being from a hacker network.
That's the root of issues with spam filtering. Making good rules. Even harder is making rules that work yesterday, today, and next week.
The sudden cease of email notifications could just be that someone started a using a new firewall rule at your provider that impacts LJ email traffic?
It is hard for an outsider without provider access to help you. The challenge is LJ uses an internet provider that sells filtered 'safe' side of internet web hosting, and even our admin do not have access to the secret firewall rules to stop network hackers.
Best I can tell, LJ site is filtered by Cloudflare hacking/bot filter services (filters billions of packets per second), probably paid for by Vertical Scope, Inc; the actual service provider hosting the forum software.
https://server.easycounter.com/lumberjocks.com
IME - Only YOUR provider is going to be able check the filtering rules, and track down your lost email notifications.
Best Luck.