Good afternoon.
I'm using the CB in the English-language version.
I may have been able to find the reason why the CB is unstable in sending registration letters.

The reason is the PHP version. With PHP 7.2, CB sends emails and works correctly.

If you mark the PHP 7.3 version on the host, then CB stops working. Specifically, CB reports that the e-mail was successfully sent, but in fact the e-mail is not. The joomla log files are empty.
Do you have CB running on PHP7.3 ? Does it work?
Thank you very much for your support.

Checked several times and confirmed that when marked on the host use PHP 7.3 or 7.4 CB works, but mail is not sent.
When using PHP 7.2 CB works correctly, mail is sent.

The technical requirements in all PHP installations follow the CB recommendations.

The host didn't give us an exact answer, but suggested that "The CB developer may be using an outdated version of phpmailer in his module, while the system kernel is using a newer version".
They encountered similar inconsistencies for wordpress with PHP 7.3 and above, which is resolved by adding the following code at the end of the wp-config.php file:

// PHPMailer PHP 7.3+ fix
add_filter( 'wp_mail', function( $mailArray ) {
global $phpmailer;
if ( ! ( $phpmailer instanceof PHPMailer ) ) {
require_once ABSPATH . WPINC . '/class-phpmailer.php';
require_once ABSPATH . WPINC . '/class-smtp.php';
$phpmailer = new PHPMailer( true );
}
$phpmailer::$validator = 'php';

return $mailArray;
} );
// PHPMailer PHP 7.3+ fix

Solutions for Joomla they do not have, but maybe this information will be useful to you that will solve this problem with PHP 7.3+ for Joomla when running CB.

We have not tried mass mailings - we just installed the site on the host.
A test mail is being sent.

An email from the contacts module is also sent.
Versions 7.3 are as follows:
7.3.15
7.4.3
See version the php: site/phpinfo.php .