When looking at the mail log of the server you see this log entry from Amavis:
Aug 3 01:00:20 el7p17 amavis: (114160-02) NOTICE: reconnecting in response to: err=2006, HY000, DBD::mysql::st execute failed: MySQL server has gone away at (eval 130) line 173.
Amavis tries to use a persistent connection to mariadb but the server has dropped the mariadb connection so Amavis has to try again to re-establish the connection.
How to Fix:
Check that wait_timeout mysql variable in your /etc/my.cnf configuration file is large enough (don't forget to also check for the value in your /etc/my.cnf.d/ directory overriding the default). It is recommended that you set the variable to its default value within the [mysqld] section:
[mysqld] wait_timeout = 28800
systemctl restart mariadb
Once you’ve made these changes, and restarted your MariaDB server, the issue should be fixed.