Index: src/wp-includes/pluggable.php
===================================================================
--- src/wp-includes/pluggable.php	(revision 48525)
+++ src/wp-includes/pluggable.php	(working copy)
@@ -2832,3 +2832,35 @@
 	}
 endif;
 
+if ( ! function_exists( 'wp_set_phpmailer_validator' ) ) :
+	/**
+	 * Sets the PHPMailer's validator to a value determined by the environment.
+	 *
+	 * The logic is the same as in the auto mode of PHPMailer 5.2.27 version.
+	 *
+	 * @param PHPMailer $phpmailer The PHPMailer object.
+	 */
+	function wp_set_phpmailer_validator( $phpmailer ) {
+		// Check this constant first so it works when extension_loaded() is disabled by safe mode.
+		// Constant was added in PHP 5.2.4.
+		if ( defined( 'PCRE_VERSION' ) ) {
+			// This pattern can get stuck in a recursive loop in PCRE <= 8.0.2.
+			if ( version_compare( PCRE_VERSION, '8.0.3' ) >= 0 ) {
+				$patternselect = 'pcre8';
+			} else {
+				$patternselect = 'pcre';
+			}
+		} elseif ( function_exists( 'extension_loaded' ) && extension_loaded( 'pcre' ) ) {
+			// Fall back to older PCRE.
+			$patternselect = 'pcre';
+		} else {
+			// Filter_var appeared in PHP 5.2.0 and does not require the PCRE extension.
+			if ( version_compare( PHP_VERSION, '5.2.0' ) >= 0 ) {
+				$patternselect = 'php';
+			} else {
+				$patternselect = 'noregex';
+			}
+		}
+		$phpmailer::$validator = $patternselect;
+	}
+endif;
Index: src/wp-includes/class-phpmailer.php
===================================================================
--- src/wp-includes/class-phpmailer.php	(revision 48525)
+++ src/wp-includes/class-phpmailer.php	(working copy)
@@ -17,3 +17,6 @@
 
 class_alias( PHPMailer\PHPMailer\PHPMailer::class, 'PHPMailer' );
 class_alias( PHPMailer\PHPMailer\Exception::class, 'phpmailerException' );
+
+wp_set_phpmailer_validator( new PHPMailer\PHPMailer\PHPMailer );
+