Try parse string multipart

app/src/main/java/eu/faircode/email/MessageHelper.java

@@ -107,6 +107,7 @@ import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
 import javax.activation.DataHandler;
+import javax.activation.DataSource;
 import javax.activation.FileDataSource;
 import javax.activation.FileTypeMap;
 import javax.mail.Address;
@@ -4051,6 +4052,10 @@ public class MessageHelper {
 
                 if (part.isMimeType("multipart/mixed")) {
                     Object content = part.getContent();
+
+                    if (content instanceof String)
+                        content = tryParseMultipart((String) content, part.getContentType());
+
                     if (content instanceof Multipart) {
                         Multipart mp = (Multipart) content;
                         for (int i = 0; i < mp.getCount(); i++) {
@@ -4074,6 +4079,10 @@ public class MessageHelper {
                             "application/pkcs7-signature".equals(protocol) ||
                             "application/x-pkcs7-signature".equals(protocol)) {
                         Object content = part.getContent();
+
+                        if (content instanceof String)
+                            content = tryParseMultipart((String) content, part.getContentType());
+
                         if (content instanceof Multipart) {
                             Multipart multipart = (Multipart) content;
                             if (multipart.getCount() == 2) {
@@ -4120,6 +4129,10 @@ public class MessageHelper {
                     String protocol = ct.getParameter("protocol");
                     if ("application/pgp-encrypted".equals(protocol) || protocol == null) {
                         Object content = part.getContent();
+
+                        if (content instanceof String)
+                            content = tryParseMultipart((String) content, part.getContentType());
+
                         if (content instanceof Multipart) {
                             Multipart multipart = (Multipart) content;
                             if (multipart.getCount() == 2) {
@@ -4218,6 +4231,10 @@ public class MessageHelper {
             if (part.isMimeType("multipart/*")) {
                 Multipart multipart;
                 Object content = part.getContent(); // Should always be Multipart
+
+                if (content instanceof String)
+                    content = tryParseMultipart((String) content, part.getContentType());
+
                 if (content instanceof Multipart) {
                     multipart = (Multipart) part.getContent();
                     int count = multipart.getCount();
@@ -4374,6 +4391,35 @@ public class MessageHelper {
         }
     }
 
+    private Object tryParseMultipart(String text, String contentType) {
+        try {
+            return new MimeMultipart(new DataSource() {
+                @Override
+                public InputStream getInputStream() throws IOException {
+                    return new ByteArrayInputStream(text.getBytes(StandardCharsets.ISO_8859_1));
+                }
+
+                @Override
+                public OutputStream getOutputStream() throws IOException {
+                    return null;
+                }
+
+                @Override
+                public String getContentType() {
+                    return contentType;
+                }
+
+                @Override
+                public String getName() {
+                    return "String";
+                }
+            });
+        } catch (MessagingException ex) {
+            Log.e(ex);
+            return text;
+        }
+    }
+
     private void ensureEnvelope() throws MessagingException {
         _ensureMessage(false, false);
     }