The way AppMail works in most cases where performance is important, is that it fetches the data from the database once, then it will store it in cache and serve it from there for as much as possible.
This allows AppMail to perform fast because data access from the cache is very fast.

However, there are times when the application cache must be cleared, one such example is after doing a AppMail version update. When this happens, AppMail will try to clear the cache by itself, so that any new changes in the new version are reflected properly. If AppMail fails to do this, for any reason, then you might need to do it manually using the clear-cache command.

An example of this situation was after an update to v2.1.3, when trying to create a campaign, some instances of AppMail got this error: "CampaignOption.email_stats_delay_days" is not defined. The v2.1.2 version introduced a new column in the database and since the cache was not properly regenerated, the old cache for the database schema was not reflecting the new column, so it was reported as not found. Clearing the cache manually fixed the problem.

The command has the following signature:

command: /usr/bin/php -q {ABSOLUTE_PATH_TO}/apps/console/console.php clear-cache --verbose=1
--verbose=1 whether to show output in the console.

So, a valid call to this command will look like:

/usr/bin/php -q /var/www/html/apps/console/console.php clear-cache --verbose=1

Since running this command will clear the application cache, the application will be slower for a while until the cache is regenerated.

You should not abuse this command, only run it when it is really needed, preferably when the application is not busy and when no campaigns are going out.