diff --git a/src/PHPMailer.php b/src/PHPMailer.php
index 26bc6cce..5e8bbe6f 100644
--- a/src/PHPMailer.php
+++ b/src/PHPMailer.php
@@ -1124,6 +1124,22 @@ class PHPMailer
         return call_user_func_array([$this, 'addAnAddress'], $params);
     }
 
+    /**
+     * Set the boundaries to use for delimiting MIME parts.
+     * If you override this, ensure you set all 3 boundaries to unique values.
+     * The default boundaries include a "=_" sequence which cannot occur in quoted-printable bodies,
+     * as suggested by https://www.rfc-editor.org/rfc/rfc2045#section-6.7
+     *
+     * @return void
+     */
+    public function setBoundaries()
+    {
+        $this->uniqueid = $this->generateId();
+        $this->boundary[1] = 'b1=_' . $this->uniqueid;
+        $this->boundary[2] = 'b2=_' . $this->uniqueid;
+        $this->boundary[3] = 'b3=_' . $this->uniqueid;
+    }
+
     /**
      * Add an address to one of the recipient arrays or to the ReplyTo array.
      * Addresses that have been added already return false, but do not throw exceptions.
@@ -2794,10 +2810,7 @@ class PHPMailer
     {
         $body = '';
         //Create unique IDs and preset boundaries
-        $this->uniqueid = $this->generateId();
-        $this->boundary[1] = 'b1_' . $this->uniqueid;
-        $this->boundary[2] = 'b2_' . $this->uniqueid;
-        $this->boundary[3] = 'b3_' . $this->uniqueid;
+        $this->setBoundaries();
 
         if ($this->sign_key_file) {
             $body .= $this->getMailMIME() . static::$LE;
@@ -3069,6 +3082,18 @@ class PHPMailer
         return $body;
     }
 
+    /**
+     * Get the boundaries that this message will use
+     * @return array
+     */
+    public function getBoundaries()
+    {
+        if (empty($this->boundary)) {
+            $this->setBoundaries();
+        }
+        return $this->boundary;
+    }
+
     /**
      * Return the start of a message boundary.
      *
diff --git a/test/PHPMailer/GenerateIdTest.php b/test/PHPMailer/GenerateIdTest.php
index 5230b4ce..c9dcf257 100644
--- a/test/PHPMailer/GenerateIdTest.php
+++ b/test/PHPMailer/GenerateIdTest.php
@@ -49,7 +49,7 @@ final class GenerateIdTest extends PreSendTestCase
         self::assertSame(
             1,
             preg_match(
-                '`Content-Type: multipart/alternative;\s+boundary="(b[1-3]_[A-Za-z0-9]{32,})"`',
+                '`Content-Type: multipart/alternative;\s+boundary="(b[1-3]=_[A-Za-z0-9]{32,})"`',
                 $message,
                 $matches
             ),
@@ -64,4 +64,12 @@ final class GenerateIdTest extends PreSendTestCase
             'No boundaries using the generated ID found in message'
         );
     }
+
+    public function testBoundaries()
+    {
+        $boundaries = $this->Mail->getBoundaries();
+        self::assertMatchesRegularExpression('/b[1-3]=_[A-Za-z0-9]{32,}/', $boundaries[1]);
+        self::assertMatchesRegularExpression('/b[1-3]=_[A-Za-z0-9]{32,}/', $boundaries[2]);
+        self::assertMatchesRegularExpression('/b[1-3]=_[A-Za-z0-9]{32,}/', $boundaries[3]);
+    }
 }