diff --git a/src/PHPMailer.php b/src/PHPMailer.php
index 5411e4ff8..16359fabe 100644
--- a/src/PHPMailer.php
+++ b/src/PHPMailer.php
@@ -4350,12 +4350,14 @@ public function DKIM_Sign($signHeader)
      */
     public function DKIM_HeaderC($signHeader)
     {
-        //Unfold all header continuation lines
-        //Also collapses folded whitespace.
         //Note PCRE \s is too broad a definition of whitespace; RFC5322 defines it as `[ \t]`
         //@see https://tools.ietf.org/html/rfc5322#section-2.2
         //That means this may break if you do something daft like put vertical tabs in your headers.
-        $signHeader = preg_replace('/\r\n[ \t]+/', ' ', $signHeader);
+        //Unfold header lines
+        $signHeader = preg_replace('/\r\n[ \t]+/m', '', $signHeader);
+        //Collapse internal whitespace to a single space
+//        $signHeader = preg_replace('/[ \t]+/', ' ', $signHeader);
+        //Break headers out into an array
         $lines = explode("\r\n", $signHeader);
         foreach ($lines as $key => $line) {
             //If the header is missing a :, skip it as it's invalid
@@ -4367,12 +4369,12 @@ public function DKIM_HeaderC($signHeader)
             list($heading, $value) = explode(':', $line, 2);
             //Lower-case header name
             $heading = strtolower($heading);
-            //Collapse white space within the value
-            $value = preg_replace('/[ \t]{2,}/', ' ', $value);
+            //Collapse white space within the value, also convert WSP to space
+            $value = preg_replace('/[ \t]+/', ' ', $value);
             //RFC6376 is slightly unclear here - it says to delete space at the *end* of each value
             //But then says to delete space before and after the colon.
             //Net result is the same as trimming both ends of the value.
-            //by elimination, the same applies to the field name
+            //By elimination, the same applies to the field name
             $lines[$key] = trim($heading, " \t") . ':' . trim($value, " \t");
         }
 
diff --git a/test/PHPMailerTest.php b/test/PHPMailerTest.php
index 66ea9ab4a..908a324ce 100644
--- a/test/PHPMailerTest.php
+++ b/test/PHPMailerTest.php
@@ -2060,6 +2060,19 @@ public function testDKIMHeaderCanonicalization()
             $this->Mail->DKIM_HeaderC($preheaders),
             'DKIM header canonicalization incorrect'
         );
+        //Check that long folded lines with runs of spaces are canonicalised properly
+        $preheaders = "Long-Header-1: <https://example.com/somescript.php?".
+            "id=1234567890&name=Abcdefghijklmnopquestuvwxyz&hash=\r\n abc1234".
+            "\r\nLong-Header-2: This  is  a  long  header  value  that  contains  runs  of  spaces and trailing    ".
+            "\r\n and   is   folded   onto   2   lines";
+        $postheaders = "long-header-1:<https://example.com/somescript.php?id=1234567890&".
+            "name=Abcdefghijklmnopquestuvwxyz&hash=abc1234\r\nlong-header-2:This is a long".
+            " header value that contains runs of spaces and trailing and is folded onto 2 lines";
+        $this->assertEquals(
+            $postheaders,
+            $this->Mail->DKIM_HeaderC($preheaders),
+            'DKIM header canonicalization of long lines incorrect'
+        );
     }
 
     /**