diff --git a/app/src/main/java/eu/faircode/email/FragmentCompose.java b/app/src/main/java/eu/faircode/email/FragmentCompose.java
index 55089edb56..fa1215750a 100644
--- a/app/src/main/java/eu/faircode/email/FragmentCompose.java
+++ b/app/src/main/java/eu/faircode/email/FragmentCompose.java
@@ -3835,7 +3835,8 @@ public class FragmentCompose extends FragmentBase {
                         };
                         bpContent.setContent(imessage.getContent(), imessage.getContentType());
 
-                        try (OutputStream out = new FileOutputStream(input)) {
+                        try (OutputStream out = new MessageHelper.CanonicalizingStream(
+                                new BufferedOutputStream(new FileOutputStream(input)))) {
                             bpContent.writeTo(out);
                         }
                     } else {
@@ -4248,8 +4249,9 @@ public class FragmentCompose extends FragmentBase {
 
                 // Build content
                 File sinput = new File(tmp, draft.id + ".smime_sign");
-                try (FileOutputStream fos = new FileOutputStream(sinput)) {
-                    bpContent.writeTo(fos);
+                try (OutputStream os = new MessageHelper.CanonicalizingStream(
+                        new BufferedOutputStream(new FileOutputStream(sinput)))) {
+                    bpContent.writeTo(os);
                 }
 
                 if (EntityMessage.SMIME_SIGNONLY.equals(type)) {
diff --git a/app/src/main/java/eu/faircode/email/MessageHelper.java b/app/src/main/java/eu/faircode/email/MessageHelper.java
index 8773d504d7..fe8ecde783 100644
--- a/app/src/main/java/eu/faircode/email/MessageHelper.java
+++ b/app/src/main/java/eu/faircode/email/MessageHelper.java
@@ -77,6 +77,7 @@ import org.simplejavamail.outlookmessageparser.model.OutlookFileAttachment;
 import org.simplejavamail.outlookmessageparser.model.OutlookMessage;
 
 import java.io.BufferedInputStream;
+import java.io.BufferedOutputStream;
 import java.io.BufferedReader;
 import java.io.ByteArrayInputStream;
 import java.io.ByteArrayOutputStream;
@@ -84,6 +85,7 @@ import java.io.File;
 import java.io.FileInputStream;
 import java.io.FileOutputStream;
 import java.io.FileReader;
+import java.io.FilterOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
@@ -4098,23 +4100,9 @@ public class MessageHelper {
             if (TextUtils.isEmpty(boundary))
                 throw new ParseException("Signed boundary missing");
 
-            ByteArrayOutputStream bos = new ByteArrayOutputStream();
-            apart.part.writeTo(bos);
-            String raw = new String(bos.toByteArray(), StandardCharsets.ISO_8859_1);
-            String[] parts = raw.split("\\r?\\n" + Pattern.quote("--" + boundary) + "\\r?\\n");
-            if (parts.length < 2)
-                throw new ParseException("Signed part missing");
-
-            // PGP: https://datatracker.ietf.org/doc/html/rfc3156#section-5
-            // S/MIME: https://datatracker.ietf.org/doc/html/rfc8551#section-3.1.1
-            String c = parts[1]
-                    .replaceAll("\\r?\\n", "\r\n"); // normalize new lines
-            if (EntityAttachment.PGP_CONTENT.equals(apart.encrypt))
-                c = c.replaceAll(" +$", ""); // trim trailing spaces
-
             File file = local.getFile(context);
-            try (OutputStream os = new FileOutputStream(file)) {
-                os.write(c.getBytes(StandardCharsets.ISO_8859_1));
+            try (OutputStream os = new BufferedOutputStream(new CanonicalizingStream(new FileOutputStream(file), boundary))) {
+                apart.part.writeTo(os);
             }
 
             DB db = DB.getInstance(context);
@@ -5771,4 +5759,104 @@ public class MessageHelper {
             return "message/feedback-report".equalsIgnoreCase(type);
         }
     }
+
+    static class CanonicalizingStream extends FilterOutputStream {
+        private OutputStream os;
+        private String boundary;
+
+        private int boundaries = 0;
+        private boolean carriage = false;
+        private ByteArrayOutputStream buffer = new ByteArrayOutputStream();
+
+        private static final byte[] CRLF = "\r\n".getBytes(StandardCharsets.ISO_8859_1);
+
+        // PGP: https://datatracker.ietf.org/doc/html/rfc3156#section-5
+        // S/MIME: https://datatracker.ietf.org/doc/html/rfc8551#section-3.1.1
+
+        public CanonicalizingStream(OutputStream out) {
+            super(out);
+            this.os = out;
+            this.boundary = null;
+        }
+
+        public CanonicalizingStream(OutputStream out, String boundary) {
+            super(out);
+            this.os = out;
+            this.boundary = "--" + boundary;
+        }
+
+        @Override
+        public void write(int b) throws IOException {
+            this.write(new byte[]{(byte) b}, 0, 1);
+        }
+
+        @Override
+        public void write(byte[] b) throws IOException {
+            this.write(b, 0, b.length);
+        }
+
+        @Override
+        public void write(byte[] b, int off, int len) throws IOException {
+            for (int i = off; i < off + len; i++) {
+                byte k = b[i];
+                if (k == '\r')
+                    carriage = true;
+                else {
+                    if (k == '\n') {
+                        if (writeBuffer())
+                            buffer.write(CRLF);
+                    } else {
+                        if (carriage) {
+                            if (writeBuffer())
+                                buffer.write(CRLF);
+                        }
+                        buffer.write(k);
+                    }
+                    carriage = false;
+                }
+            }
+        }
+
+        @Override
+        public void flush() throws IOException {
+            flushBuffer();
+            super.flush();
+        }
+
+        @Override
+        public void close() throws IOException {
+            flushBuffer();
+            super.close();
+        }
+
+        private boolean writeBuffer() throws IOException {
+            try {
+                String line = new String(buffer.toByteArray(), StandardCharsets.ISO_8859_1);
+
+                if (boundary != null) {
+                    if (boundary.equals(line.trim())) {
+                        boundaries++;
+                        return false;
+                    }
+                    if (boundaries != 1)
+                        return false;
+                }
+
+                line = line.replaceAll(" +$", "");
+
+                os.write(line.getBytes(StandardCharsets.ISO_8859_1));
+
+                return true;
+            } finally {
+                buffer.reset();
+            }
+        }
+
+        private void flushBuffer() throws IOException {
+            if (boundary != null && boundaries != 2)
+                throw new IOException("Content boundaries=" + boundaries);
+            if (buffer.size() > 0)
+                writeBuffer();
+        }
+    }
 }