diff --git a/app/src/main/java/eu/faircode/email/MessageHelper.java b/app/src/main/java/eu/faircode/email/MessageHelper.java
index 105af38460..7ec9a55dc9 100644
--- a/app/src/main/java/eu/faircode/email/MessageHelper.java
+++ b/app/src/main/java/eu/faircode/email/MessageHelper.java
@@ -1475,6 +1475,8 @@ public class MessageHelper {
                         attachmentPart.setDisposition(attachment.disposition);
                     if (attachment.cid != null)
                         attachmentPart.setHeader("Content-ID", attachment.cid);
+                    if ("message/rfc822".equals(attachment.type))
+                        attachmentPart.setHeader("Content-Transfer-Encoding", "base64");
 
                     if (attachment.isInline())
                         relatedMultiPart.addBodyPart(attachmentPart);