diff --git a/src/wp-includes/formatting.php b/src/wp-includes/formatting.php
index e46e5d82ad590..8d9e6656e8718 100644
--- a/src/wp-includes/formatting.php
+++ b/src/wp-includes/formatting.php
@@ -6171,13 +6171,21 @@ function wp_staticize_emoji_for_email( $mail ) {
 		}
 	}
 
-	foreach ( $headers as $header ) {
-		if ( ! str_contains( $header, ':' ) ) {
+	foreach ( $headers as $key => $header ) {
+		if ( is_string( $header ) &&
+			strpos( $header, ':' ) === false &&
+			is_numeric( $key )
+		) {
 			continue;
 		}
 
 		// Explode them out.
-		list( $name, $content ) = explode( ':', trim( $header ), 2 );
+		list( $name, $content ) = ( is_numeric( $key ) ) ? explode( ':', trim( $header ), 2 ) : array( $key, $header );
+
+		// Skip because Content-Type must be an string.
+		if ( ! is_string( $name ) || ! is_string( $content ) ) {
+			continue;
+		}
 
 		// Cleanup crew.
 		$name    = trim( $name );
diff --git a/src/wp-includes/pluggable.php b/src/wp-includes/pluggable.php
index 5edd0c760cbb2..0462539936b64 100644
--- a/src/wp-includes/pluggable.php
+++ b/src/wp-includes/pluggable.php
@@ -306,16 +306,20 @@ function wp_mail( $to, $subject, $message, $headers = '', $attachments = array()
 			// If it's actually got contents.
 			if ( ! empty( $tempheaders ) ) {
 				// Iterate through the raw headers.
-				foreach ( (array) $tempheaders as $header ) {
-					if ( ! str_contains( $header, ':' ) ) {
+				foreach ( (array) $tempheaders as $key => $header ) {
+					if ( is_array( $header ) ) {
+						$header = implode( ',', $header );
+					} elseif ( strpos( $header, ':' ) === false ) {
 						if ( false !== stripos( $header, 'boundary=' ) ) {
 							$parts    = preg_split( '/boundary=/i', trim( $header ) );
 							$boundary = trim( str_replace( array( "'", '"' ), '', $parts[1] ) );
 						}
-						continue;
+						if ( is_numeric( $key ) ) {
+							continue;
+						}
 					}
 					// Explode them out.
-					list( $name, $content ) = explode( ':', trim( $header ), 2 );
+					list( $name, $content ) = ( is_numeric( $key ) ) ? explode( ':', trim( $header ), 2 ) : array( $key, $header );
 
 					// Cleanup crew.
 					$name    = trim( $name );
@@ -359,17 +363,34 @@ function wp_mail( $to, $subject, $message, $headers = '', $attachments = array()
 							}
 							break;
 						case 'cc':
-							$cc = array_merge( (array) $cc, explode( ',', $content ) );
+							$cc = array_merge(
+								(array) $cc,
+								( is_array( $content ) ) ? $content : explode( ',', $content )
+							);
 							break;
 						case 'bcc':
-							$bcc = array_merge( (array) $bcc, explode( ',', $content ) );
+							$bcc = array_merge(
+								(array) $bcc,
+								( is_array( $content ) ) ? $content : explode( ',', $content )
+							);
 							break;
 						case 'reply-to':
-							$reply_to = array_merge( (array) $reply_to, explode( ',', $content ) );
+							$reply_to = array_merge(
+								(array) $reply_to,
+								( is_array( $content ) ) ? $content : explode( ',', $content )
+							);
 							break;
 						default:
-							// Add it to our grand headers array.
-							$headers[ trim( $name ) ] = trim( $content );
+							$name    = trim( $name );
+							$content = trim( $content );
+							if ( isset( $headers[ $name ] ) ) {
+								if ( ! is_array( $headers[ $name ] ) ) {
+									$headers[ $name ] = array( $headers[ $name ] );
+								}
+								$headers[ $name ][] = $content;
+							} else {
+								$headers[ $name ] = $content;
+							}
 							break;
 					}
 				}
@@ -532,7 +553,13 @@ function wp_mail( $to, $subject, $message, $headers = '', $attachments = array()
 				// Only add custom headers not added automatically by PHPMailer.
 				if ( ! in_array( $name, array( 'MIME-Version', 'X-Mailer' ), true ) ) {
 					try {
-						$phpmailer->addCustomHeader( sprintf( '%1$s: %2$s', $name, $content ) );
+						if ( is_array( $content ) ) {
+							foreach ( $content as $value ) {
+								$phpmailer->addCustomHeader( sprintf( '%1$s: %2$s', $name, $value ) );
+							}
+						} else {
+							$phpmailer->addCustomHeader( sprintf( '%1$s: %2$s', $name, $content ) );
+						}
 					} catch ( PHPMailer\PHPMailer\Exception $e ) {
 						continue;
 					}
diff --git a/tests/phpunit/tests/pluggable/wpMail.php b/tests/phpunit/tests/pluggable/wpMail.php
index 85e3b46f61a5b..74aec391cffa1 100644
--- a/tests/phpunit/tests/pluggable/wpMail.php
+++ b/tests/phpunit/tests/pluggable/wpMail.php
@@ -555,6 +555,89 @@ public function test_wp_mail_resets_properties() {
 		$this->assertNotSame( 'user1', $phpmailer->AltBody );
 	}
 
+	/**
+	 * Test that multiple headers with the same name are sent correctly.
+	 *
+	 * @ticket 30128
+	 * @ticket 56779
+	 */
+	public function test_wp_mail_with_multiple_same_name_headers() {
+		$to      = 'test@example.com';
+		$subject = 'Testing Multiple Headers';
+		$message = 'This is a test message.';
+		$headers = array(
+			'x-my-things: thing1',
+			'x-my-things: thing2',
+		);
+
+		wp_mail( $to, $subject, $message, $headers );
+
+		$mailer      = tests_retrieve_phpmailer_instance();
+		$sent_header = $mailer->get_sent()->header;
+
+		$this->assertStringContainsString( 'x-my-things: thing1', $sent_header, 'The first header was not sent.' );
+		$this->assertStringContainsString( 'x-my-things: thing2', $sent_header, 'The second header was not sent.' );
+	}
+
+	/**
+	 * Tests that headers are handled correctly in various formats.
+	 *
+	 * @dataProvider data_wp_mail_header_formats
+	 *
+	 * @ticket 30128
+	 */
+	public function test_wp_mail_header_formats( $headers, $expected_strings ) {
+		wp_mail( 'test@example.com', 'Test', 'Message', $headers );
+
+		$mailer      = tests_retrieve_phpmailer_instance();
+		$sent_header = $mailer->get_sent()->header;
+
+		foreach ( $expected_strings as $expected_string ) {
+			$this->assertStringContainsString( $expected_string, $sent_header );
+		}
+	}
+
+	/**
+	 * Data provider for test_wp_mail_header_formats.
+	 */
+	public function data_wp_mail_header_formats() {
+		return array(
+			'associative array'                   => array(
+				'headers'          => array(
+					'From' => 'Me Myself <me@example.org>',
+					'Cc'   => 'Isaaco Harrelson <iharrel@example.org>',
+				),
+				'expected_strings' => array(
+					'From: Me Myself <me@example.org>',
+					'Cc: Isaaco Harrelson <iharrel@example.org>',
+				),
+			),
+			'indexed array'                       => array(
+				'headers'          => array(
+					'From: Me Myself <me@example.org>',
+					'Cc: Isaaco Harrelson <iharrel@example.org>',
+				),
+				'expected_strings' => array(
+					'From: Me Myself <me@example.org>',
+					'Cc: Isaaco Harrelson <iharrel@example.org>',
+				),
+			),
+			'associative array with multiple CCs' => array(
+				'headers'          => array(
+					'From' => 'Me Myself <me@example.org>',
+					'Cc'   => array(
+						'Isaaco Harrelson <iharrel@example.org>',
+						'frederick@wordpress.org',
+					),
+				),
+				'expected_strings' => array(
+					'From: Me Myself <me@example.org>',
+					'Cc: Isaaco Harrelson <iharrel@example.org>, frederick@wordpress.org',
+				),
+			),
+		);
+	}
+
 	/**
 	 * Tests that wp_mail() can send embedded images.
 	 *