diff --git a/wp-includes/pluggable.php b/wp-includes/pluggable.php
index 9336e03c7e..835a0d6df3 100644
--- a/wp-includes/pluggable.php
+++ b/wp-includes/pluggable.php
@@ -450,10 +450,13 @@ if ( ! function_exists( 'wp_mail' ) ) :
 		 */
 		$phpmailer->CharSet = apply_filters( 'wp_mail_charset', $charset );
 
-		// Set custom headers
+		// Set custom headers.
 		if ( ! empty( $headers ) ) {
 			foreach ( (array) $headers as $name => $content ) {
-				$phpmailer->addCustomHeader( sprintf( '%1$s: %2$s', $name, $content ) );
+				// Only add custom headers not added automatically by PHPMailer.
+				if ( ! in_array( $name, array( 'MIME-Version', 'X-Mailer') ) ) {
+					$phpmailer->addCustomHeader( sprintf( '%1$s: %2$s', $name, $content ) );
+				}
 			}
 
 			if ( false !== stripos( $content_type, 'multipart' ) && ! empty( $boundary ) ) {
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 8f4f2f8df3..a000f98723 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.3-alpha-46114';
+$wp_version = '5.3-alpha-46115';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.