spamd fails with "MIRRORED.BY file contents were missing, channel failed" error

Symptoms

There are occasions where spamd fails to start with the following error: 

# systemctl status spamd
...
server.cpanel.tld systemd[1]: Starting Apache SpamAssassin™ deferral daemon...
server.cpanel.tld spamd[20813]: config: no rules were found! Do you need to run 'sa-update'?

However, attempting to run the recommended sa-update command to resolve this results in the following error:

# /usr/local/cpanel/3rdparty/bin/sa-update
error: no mirror data available for channel updates.spamassassin.org
channel 'updates.spamassassin.org': MIRRORED.BY file contents were missing, channel failed

Cause

SpamAssassin is unable to start due to rules being missing; meanwhile, the update-sa command is unable to complete its tasks to repair the SpamAssassin ruleset as a critical file named "MIRROR.BY" is missing. This file is typically automatically downloaded by update-sa if missing, however, it is possible to become rate-limited by SpamAssassin's update mirrors causing this file to be unavailable from the mirrors during the previous update.

Resolution 

update-sa needs this file to be able to generate the missing ruleset, and thus will need to be manually downloaded in order to resolve this.

1. Access the server via SSH or via the Terminal in WHM.

2. Navigate to the SpamAssassin versioned folder using the following command:

   # cd /var/lib/spamassassin/$(rpm -q --queryformat "%{VERSION}" `rpm -qa | grep -Ei "cpanel-.*-Mail-SpamAssassin"`;)/updates_spamassassin_org

3. Move aside any current MIRRORED.BY file, if any:

   # mv -v MIRRORED.BY{,.cpbak}

4. Download the updated MIRRORED.BY file to this location:

   # wget https://spamassassin.apache.org/updates/MIRRORED.BY

   Note: If you receive an error when attempting to perform the above, it indicates that you are rate-limited by SpamAssassin servers. The file will need to be downloaded using a different device, and then manually copied to this server. 

5. Run a SpamAssassin update:

   # /usr/local/cpanel/3rdparty/bin/sa-update -vvv

6. Finally, restart the SpamAssassin service:

   # /scripts/restartsrv_spamd