diff --git a/src/com/fsck/k9/activity/MessageView.java b/src/com/fsck/k9/activity/MessageView.java
index c93ca83b5..5e1f912dd 100644
--- a/src/com/fsck/k9/activity/MessageView.java
+++ b/src/com/fsck/k9/activity/MessageView.java
@@ -1123,12 +1123,28 @@ public class MessageView extends K9Activity implements OnClickListener {
                 return;
             }
             MessageView.this.mMessage = message;
+
+            /*
+             * Clone the message object because the original could be modified by
+             * MessagingController later. This could lead to a ConcurrentModificationException
+             * when that same object is accessed by the UI thread (below).
+             *
+             * See issue 3953
+             *
+             * This is just an ugly hack to get rid of the most pressing problem. A proper way to
+             * fix this is to make Message thread-safe. Or, even better, rewriting the UI code to
+             * access messages via a ContentProvider.
+             *
+             */
+            final Message clonedMessage = message.clone();
+
             runOnUiThread(new Runnable() {
                 public void run() {
-                    if (!message.isSet(Flag.X_DOWNLOADED_FULL) && !message.isSet(Flag.X_DOWNLOADED_PARTIAL)) {
+                    if (!clonedMessage.isSet(Flag.X_DOWNLOADED_FULL) &&
+                            !clonedMessage.isSet(Flag.X_DOWNLOADED_PARTIAL)) {
                         mMessageView.loadBodyFromUrl("file:///android_asset/downloading.html");
                     }
-                    mMessageView.setHeaders(message, account);
+                    mMessageView.setHeaders(clonedMessage, account);
                     mMessageView.setOnFlagListener(new OnClickListener() {
                         @Override
                         public void onClick(View v) {
diff --git a/src/com/fsck/k9/mail/Message.java b/src/com/fsck/k9/mail/Message.java
index 1afa5ba91..2a7cd5efd 100644
--- a/src/com/fsck/k9/mail/Message.java
+++ b/src/com/fsck/k9/mail/Message.java
@@ -224,4 +224,32 @@ public abstract class Message implements Part, Body {
         return 0;
     }
 
+    /**
+     * Copy the contents of this object into another {@code Message} object.
+     *
+     * @param destination
+     *         The {@code Message} object to receive the contents of this instance.
+     */
+    protected void copy(Message destination) {
+        destination.mUid = mUid;
+        destination.mInternalDate = mInternalDate;
+        destination.mFolder = mFolder;
+        destination.mReference = mReference;
+
+        // mFlags contents can change during the object lifetime, so copy the Set
+        destination.mFlags = new HashSet<Flag>(mFlags);
+    }
+
+    /**
+     * Creates a new {@code Message} object with the same content as this object.
+     *
+     * <p>
+     * <strong>Note:</strong>
+     * This method was introduced as a hack to prevent {@code ConcurrentModificationException}s. It
+     * shouldn't be used unless absolutely necessary. See the comment in
+     * {@link com.fsck.k9.activity.MessageView.Listener#loadMessageForViewHeadersAvailable(com.fsck.k9.Account, String, String, Message)}
+     * for more information.
+     * </p>
+     */
+    public abstract Message clone();
 }
diff --git a/src/com/fsck/k9/mail/internet/MimeHeader.java b/src/com/fsck/k9/mail/internet/MimeHeader.java
index 20002feb3..2b87416da 100644
--- a/src/com/fsck/k9/mail/internet/MimeHeader.java
+++ b/src/com/fsck/k9/mail/internet/MimeHeader.java
@@ -133,9 +133,9 @@ public class MimeHeader {
     }
 
     static class Field {
-        String name;
+        final String name;
 
-        String value;
+        final String value;
 
         public Field(String name, String value) {
             this.name = name;
@@ -153,4 +153,13 @@ public class MimeHeader {
     public void setCharset(String charset) {
         mCharset = charset;
     }
+
+    public MimeHeader clone() {
+        MimeHeader header = new MimeHeader();
+        header.mCharset = mCharset;
+
+        header.mFields = new ArrayList<Field>(mFields);
+
+        return header;
+    }
 }
diff --git a/src/com/fsck/k9/mail/internet/MimeMessage.java b/src/com/fsck/k9/mail/internet/MimeMessage.java
index 92f7b5b4e..0ac949524 100644
--- a/src/com/fsck/k9/mail/internet/MimeMessage.java
+++ b/src/com/fsck/k9/mail/internet/MimeMessage.java
@@ -553,4 +553,38 @@ public class MimeMessage extends Message {
             throw new UnsupportedOperationException("Not supported");
         }
     }
+
+    /**
+     * Copy the contents of this object into another {@code MimeMessage} object.
+     *
+     * @param message
+     *         The {@code MimeMessage} object to receive the contents of this instance.
+     */
+    protected void copy(MimeMessage message) {
+        super.copy(message);
+
+        message.mHeader = mHeader.clone();
+
+        message.mBody = mBody;
+        message.mMessageId = mMessageId;
+        message.mSentDate = mSentDate;
+        message.mDateFormat = mDateFormat;
+        message.mSize = mSize;
+
+        // These arrays are not supposed to be modified, so it's okay to reuse the references
+        message.mFrom = mFrom;
+        message.mTo = mTo;
+        message.mCc = mCc;
+        message.mBcc = mBcc;
+        message.mReplyTo = mReplyTo;
+        message.mReferences = mReferences;
+        message.mInReplyTo = mInReplyTo;
+    }
+
+    @Override
+    public MimeMessage clone() {
+        MimeMessage message = new MimeMessage();
+        copy(message);
+        return message;
+    }
 }
diff --git a/src/com/fsck/k9/mail/store/LocalStore.java b/src/com/fsck/k9/mail/store/LocalStore.java
index 13b4c816e..fbcfafa21 100644
--- a/src/com/fsck/k9/mail/store/LocalStore.java
+++ b/src/com/fsck/k9/mail/store/LocalStore.java
@@ -3310,6 +3310,25 @@ public class LocalStore extends Store implements Serializable {
                 loadHeaders();
             return super.getHeaderNames();
         }
+
+        @Override
+        public LocalMessage clone() {
+            LocalMessage message = new LocalMessage();
+            super.copy(message);
+
+            message.mId = mId;
+            message.mAttachmentCount = mAttachmentCount;
+            message.mSubject = mSubject;
+            message.mPreview = mPreview;
+            message.mToMeCalculated = mToMeCalculated;
+            message.mCcMeCalculated = mCcMeCalculated;
+            message.mToMe = mToMe;
+            message.mCcMe = mCcMe;
+            message.mHeadersLoaded = mHeadersLoaded;
+            message.mMessageDirty = mMessageDirty;
+
+            return message;
+        }
     }
 
     public static class LocalAttachmentBodyPart extends MimeBodyPart {