diff --git a/wp-includes/pluggable.php b/wp-includes/pluggable.php
index 0c61a6f7e1..ca85a85e4e 100644
--- a/wp-includes/pluggable.php
+++ b/wp-includes/pluggable.php
@@ -174,6 +174,7 @@ if ( ! function_exists( 'wp_mail' ) ) :
 	 * @since 5.5.0 is_email() is used for email validation,
 	 *              instead of PHPMailer's default validator.
 	 * @since 6.9.0 Added $embeds parameter.
+	 * @since 6.9.0 Improved Content-Type header handling for multipart messages.
 	 *
 	 * @global PHPMailer\PHPMailer\PHPMailer $phpmailer
 	 *
@@ -351,6 +352,9 @@ if ( ! function_exists( 'wp_mail' ) ) :
 								} elseif ( false !== stripos( $charset_content, 'boundary=' ) ) {
 									$boundary = trim( str_replace( array( 'BOUNDARY=', 'boundary=', '"' ), '', $charset_content ) );
 									$charset  = '';
+									if ( preg_match( '~^multipart/(\S+)~', $content_type, $matches ) ) {
+										$content_type = 'multipart/' . strtolower( $matches[1] ) . '; boundary="' . $boundary . '"';
+									}
 								}
 
 								// Avoid setting an empty $content_type.
@@ -547,10 +551,6 @@ if ( ! function_exists( 'wp_mail' ) ) :
 					}
 				}
 			}
-
-			if ( false !== stripos( $content_type, 'multipart' ) && ! empty( $boundary ) ) {
-				$phpmailer->addCustomHeader( sprintf( 'Content-Type: %s; boundary="%s"', $content_type, $boundary ) );
-			}
 		}
 
 		if ( ! empty( $attachments ) ) {
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 8c36d21d70..7bfd8a2350 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.9-beta4-61200';
+$wp_version = '6.9-beta4-61201';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.