Add methods to update or clear custom headers by name.

src/PHPMailer.php

@@ -4058,6 +4058,30 @@ class PHPMailer
         $this->CustomHeader = [];
     }
 
+    /**
+     * Clear a specific custom header.
+     */
+    public function clearCustomHeader($name)
+    {
+        foreach ($this->CustomHeader as $k => $pair) {
+            if ($pair[0] == $name) {
+                unset($this->CustomHeader[$k]);
+            }
+        }
+    }
+
+    /**
+     * Replace a custom header.
+     */
+    public function replaceCustomHeader($name, $value)
+    {
+        foreach ($this->CustomHeader as $k => $pair) {
+            if ($pair[0] == $name) {
+                $this->CustomHeader[$k] = [$name, $value];
+            }
+        }
+    }
+
     /**
      * Add an error message to the error container.
      *

test/PHPMailer/CustomHeaderTest.php

@@ -190,6 +190,23 @@ final class CustomHeaderTest extends TestCase
         ];
     }
 
+    /**
+     * Test removing previously set custom header.
+     *
+     * @covers \PHPMailer\PHPMailer\PHPMailer::clearCustomHeader
+     */
+    public function testClearCustomHeader()
+    {
+        $this->Mail->addCustomHeader('foo', 'bar');
+        self::assertSame([['foo', 'bar']], $this->Mail->getCustomHeaders());
+
+        $this->Mail->clearCustomHeader('foo');
+
+        $cleared = $this->Mail->getCustomHeaders();
+        self::assertIsArray($cleared);
+        self::assertEmpty($cleared);
+    }
+
     /**
      * Test removing previously set custom headers.
      *
@@ -222,4 +239,22 @@ final class CustomHeaderTest extends TestCase
         $mail = new PHPMailer(true);
         $mail->addCustomHeader('SomeHeader', "Some\n Value");
     }
+
+    /**
+     * Test replacing a previously set custom header.
+     *
+     * @covers \PHPMailer\PHPMailer\PHPMailer::replaceCustomHeader
+     */
+    public function testReplaceCustomHeader()
+    {
+        $this->Mail->addCustomHeader('foo', 'bar');
+        $this->Mail->replaceCustomHeader('foo', 'baz');
+
+        // Test retrieving the custom header(s) and verify the updated value is seet.
+        self::assertSame(
+            [['foo', 'baz']],
+            $this->Mail->getCustomHeaders(),
+            'Custom header does not contain expected update'
+        );
+    }
 }