Today (August 9th, 2019, starting in the early morning) I noticed that numerous Sophos UTM firewalls were sending the notification “The spam filter daemon is unable to reach the database servers via HTTP. Please make sure that the device is able to send HTTP (TCP port 80) requests to the Internet. You may have to allow such traffic on upstream devices.”.
Everything is still functioning and upon troubleshooting I noticed that nothing had been changed, nor was broken. I believe this is a service outage of some type.
This issue has also been reported by numerous other users here: https://community.sophos.com/products/unified-threat-management/f/mail-protection-smtp-pop3-antispam-and-antivirus/114516/getting-spam-filter-cannot-query/411525
I will be updating this post with more information, leave a comment if you have information, or if the issue is also happening to you!