Hi,
You will find this code in the file components/com_acymailing/inc/phpmailer/class.phpmailer.php
Find this code:
$result .= $this->HeaderLine('X-Mailer', 'PHPMailer (phpmailer.sourceforge.net) [version ' . $this->Version . ']');
You can delete this line to remove the X-Mailer PHPMailer signature.