I followed the Smoothwall KB. It resolved the issue for site https://www.s4snextgen.org/ however still get the certificate error for https://accessitsoftware.com/updates
Another way to solve this is create a rule in the Exchange Admin Centre in Office 365.
In mail flow you could make a rule along the lines of: if email sent to group, delete, except if is received from…
Microsoft have released an update so you don't need the May update to be installed first.
For any missing the May or later updates install the standalone 10 August 2021 SSU - KB5005260. I've not tested this but the update is showing in my WSUS
Do your computers use a proxy server for internet connection? In the past I had to add an exception for the WSUS server in internet settings on the client computers.
http://wsusserver:8530 or https://wsusserver:8531
I did this is the past but only by disabling search from the start menu. If I remember correctly a user gpo with a software restriction policy to disable access to:
C:\Windows\SystemApps\Microsoft.Windows.Cortana_cw5n1h2txyewy
Discovered this as well. I have two logins. One for a secondary school and one for a primary school.
If I login with the primary school account while on the secondary school network I get the secondary school filter lists.
Also a Suffolk school with similar issues since the Smoothwall migration. Using proxy / port to set filtering levels. I have a number of tickets open with County and have had particular issues with Renaissance Learning website not responding this week.