diff --git a/dkim/dkim.go b/dkim/dkim.go
index 7a71c94..134b23d 100644
--- a/dkim/dkim.go
+++ b/dkim/dkim.go
@@ -9,6 +9,7 @@ import (
 	"bytes"
 	"crypto"
 	"crypto/ed25519"
+	"crypto/rand"
 	"crypto/x509"
 	"encoding/pem"
 	"errors"
@@ -75,14 +76,19 @@ func NewFromEd25519Key(k []byte, sc *SignerConfig) (*Middleware, error) {
 
 // Handle is the handler method that satisfies the mail.Middleware interface
 func (d Middleware) Handle(m *mail.Msg) *mail.Msg {
-	ibuf := bytes.Buffer{}
-	_, err := m.WriteToSkipMiddleware(&ibuf, Type)
+	// If no boundary is set for the mail.Msg we need to set our own fixed boundary, otherwise
+	// a new boundary will bet generated after the middleware has been applied and therfore
+	// the body hash will be altered
+	// TODO: Add a GetBoundary() method to go-mail, so we don't override a already set boundary
+	m.SetBoundary(randomBoundary())
+	ibuf := bytes.NewBuffer(nil)
+	_, err := m.WriteToSkipMiddleware(ibuf, Type)
 	if err != nil {
 		return m
 	}
 
 	var obuf bytes.Buffer
-	if err := dkim.Sign(&obuf, &ibuf, d.so); err != nil {
+	if err := dkim.Sign(&obuf, ibuf, d.so); err != nil {
 		return m
 	}
 	br := bufio.NewReader(&obuf)
@@ -157,3 +163,13 @@ func extractDKIMHeader(br *bufio.Reader) (string, error) {
 	}
 	return "", nil
 }
+
+// randomBoundary generates boundary in case no boundary is set yet
+func randomBoundary() string {
+	var buf [30]byte
+	_, err := io.ReadFull(rand.Reader, buf[:])
+	if err != nil {
+		panic(err)
+	}
+	return fmt.Sprintf("%x", buf[:])
+}
diff --git a/go.mod b/go.mod
index b830432..1d19b27 100644
--- a/go.mod
+++ b/go.mod
@@ -9,6 +9,6 @@ go 1.16
 require (
 	github.com/ProtonMail/gopenpgp/v2 v2.7.5
 	github.com/emersion/go-msgauth v0.6.8
-	github.com/wneessen/go-mail v0.4.1
+	github.com/wneessen/go-mail v0.4.2
 	golang.org/x/text v0.16.0
 )
diff --git a/go.sum b/go.sum
index 1f531ac..9bddd4e 100644
--- a/go.sum
+++ b/go.sum
@@ -27,8 +27,8 @@ github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+
 github.com/stretchr/testify v1.3.0/go.mod h1:M5WIy9Dh21IEIfnGCwXGc5bZfKNJtfHm1UVUgZn+9EI=
 github.com/stretchr/testify v1.7.0 h1:nwc3DEeHmmLAfoZucVR881uASk0Mfjw8xYJ99tb5CcY=
 github.com/stretchr/testify v1.7.0/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
-github.com/wneessen/go-mail v0.4.1 h1:m2rSg/sc8FZQCdtrV5M8ymHYOFrC6KJAQAIcgrXvqoo=
-github.com/wneessen/go-mail v0.4.1/go.mod h1:zxOlafWCP/r6FEhAaRgH4IC1vg2YXxO0Nar9u0IScZ8=
+github.com/wneessen/go-mail v0.4.2 h1:wISuU9LOGqrA7pxy7OipRtwoExXTzuGKmAjb8gYwc00=
+github.com/wneessen/go-mail v0.4.2/go.mod h1:zxOlafWCP/r6FEhAaRgH4IC1vg2YXxO0Nar9u0IScZ8=
 github.com/yuin/goldmark v1.4.13/go.mod h1:6yULJ656Px+3vBD8DxQVa3kxgyrAnzto9xy5taEt/CY=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/crypto v0.0.0-20210921155107-089bfa567519/go.mod h1:GvvjBRRGRdwPK5ydBHafDWAxML/pGHZbMvKqRZ5+Abc=