Problem with sending e-mail

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.

CB is tested up to PHP 7.4.2 as working fine. Highest PHP compatibility version is always noted in our technical requirements below.

www.joomlapolis.com/documentation/276-community-builder/getting-started/18576-technical-requirements-for-community-builder

Note we can't account for potential issues between minor PHP releases. For example if PHP 7.3.0 has a bug fixed in 7.3.1 that could be causing issues we do not account for that. It's typically best to use the highest tested version when possible.

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The host didn't give us an exact answer, but suggested that "The CB developer may be using an outdated version of phpmailer in his module, while the system kernel is using a newer version".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.

Are you able to send emails to yourself using the mass mailer in CB > User Management? Does Joomlas test email send in Joomla global configuration? I'm not able to confirm any email issues on PHP 7.4.2. What version of PHP 7.4.X are you using (the minor version number is important)? We're using PHPMailer version 5.2.28, which isn't very old and is compatible with PHP 7.4 and was released just a few months ago; we can not update to latest master version of 6.x at this time as it has massive backwards compatibility breaks.

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 .

I don't know what to suggest. We're using an up to date build of PHPMailer 5 and I don't see any breaking changes in PHP 7.4.3 so although it's not tested (we've only tested 7.4.2) it should work fine. Try adding the Port to the Host like the following.

Host: url:port

Example GMail SMTP configuration as follows.

www.joomlapolis.com/forum/149-usage/241765-smtp-connect-failed#315164

Have you tried PHP Mail instead of SMTP? Joomla will fallback to PHP Mail if SMTP fails. CB will not. CB will simply fail instead of giving the impression that it's working, but you'd typically see an error if the SMTP connection failed.