diff --git a/src/com/fsck/k9/controller/MessagingController.java b/src/com/fsck/k9/controller/MessagingController.java
index 5e1ff4956..e09453e9b 100644
--- a/src/com/fsck/k9/controller/MessagingController.java
+++ b/src/com/fsck/k9/controller/MessagingController.java
@@ -2410,7 +2410,7 @@ public class MessagingController implements Runnable {
             if (messages.isEmpty()) {
                 return;
             }
-            remoteFolder.setFlags(messages, Collections.singletonList(flag), newState);
+            remoteFolder.setFlags(messages, Collections.singleton(flag), newState);
         } finally {
             closeFolder(remoteFolder);
         }
@@ -2602,7 +2602,7 @@ public class MessagingController implements Runnable {
                 return;
             }
 
-            remoteFolder.setFlags(Collections.singletonList(Flag.SEEN), true);
+            remoteFolder.setFlags(Collections.singleton(Flag.SEEN), true);
             remoteFolder.close();
         } catch (UnsupportedOperationException uoe) {
             Log.w(K9.LOG_TAG, "Could not mark all server-side as read because store doesn't support operation", uoe);
@@ -2878,7 +2878,7 @@ public class MessagingController implements Runnable {
             }
 
             // Update the messages in the local store
-            localFolder.setFlags(messages, Collections.singletonList(flag), newState);
+            localFolder.setFlags(messages, Collections.singleton(flag), newState);
 
             int unreadMessageCount = localFolder.getUnreadMessageCount();
             for (MessagingListener l : getListeners()) {
diff --git a/src/com/fsck/k9/mail/Folder.java b/src/com/fsck/k9/mail/Folder.java
index 7d838ea43..88af1a31a 100644
--- a/src/com/fsck/k9/mail/Folder.java
+++ b/src/com/fsck/k9/mail/Folder.java
@@ -130,10 +130,10 @@ public abstract class Folder {
         }
     }
 
-    public abstract void setFlags(List<? extends Message> messages, Collection<Flag> flags, boolean value)
+    public abstract void setFlags(List<? extends Message> messages, Set<Flag> flags, boolean value)
     throws MessagingException;
 
-    public abstract void setFlags(Collection<Flag> flags, boolean value) throws MessagingException;
+    public abstract void setFlags(Set<Flag> flags, boolean value) throws MessagingException;
 
     public abstract String getUidFromMessageId(Message message) throws MessagingException;
 
diff --git a/src/com/fsck/k9/mail/Message.java b/src/com/fsck/k9/mail/Message.java
index d09c7b1a3..5a4d7a34a 100644
--- a/src/com/fsck/k9/mail/Message.java
+++ b/src/com/fsck/k9/mail/Message.java
@@ -210,8 +210,8 @@ public abstract class Message implements Part, CompositeBody {
     /*
      * TODO Refactor Flags at some point to be able to store user defined flags.
      */
-    public Collection<Flag> getFlags() {
-        return Collections.unmodifiableCollection(mFlags);
+    public Set<Flag> getFlags() {
+        return Collections.unmodifiableSet(mFlags);
     }
 
     /**
@@ -235,7 +235,7 @@ public abstract class Message implements Part, CompositeBody {
      * @param flags
      * @param set
      */
-    public void setFlags(final Collection<Flag> flags, boolean set) throws MessagingException {
+    public void setFlags(final Set<Flag> flags, boolean set) throws MessagingException {
         for (Flag flag : flags) {
             setFlag(flag, set);
         }
diff --git a/src/com/fsck/k9/mail/store/ImapStore.java b/src/com/fsck/k9/mail/store/ImapStore.java
index acfe04aa2..9042f9d9b 100644
--- a/src/com/fsck/k9/mail/store/ImapStore.java
+++ b/src/com/fsck/k9/mail/store/ImapStore.java
@@ -2110,7 +2110,7 @@ public class ImapStore extends Store {
 
 
         @Override
-        public void setFlags(Collection<Flag> flags, boolean value)
+        public void setFlags(Set<Flag> flags, boolean value)
         throws MessagingException {
             open(OPEN_MODE_RW);
             checkOpen();
@@ -2145,7 +2145,7 @@ public class ImapStore extends Store {
 
 
         @Override
-        public void setFlags(List<? extends Message> messages, final Collection<Flag> flags, boolean value)
+        public void setFlags(List<? extends Message> messages, final Set<Flag> flags, boolean value)
         throws MessagingException {
             open(OPEN_MODE_RW);
             checkOpen();
@@ -2911,7 +2911,7 @@ public class ImapStore extends Store {
         @Override
         public void setFlag(Flag flag, boolean set) throws MessagingException {
             super.setFlag(flag, set);
-            mFolder.setFlags(Collections.singletonList(this), Collections.singletonList(flag), set);
+            mFolder.setFlags(Collections.singletonList(this), Collections.singleton(flag), set);
         }
 
         @Override
diff --git a/src/com/fsck/k9/mail/store/Pop3Store.java b/src/com/fsck/k9/mail/store/Pop3Store.java
index 0a2ca179a..5e2ee632a 100644
--- a/src/com/fsck/k9/mail/store/Pop3Store.java
+++ b/src/com/fsck/k9/mail/store/Pop3Store.java
@@ -977,12 +977,12 @@ public class Pop3Store extends Store {
         }
 
         @Override
-        public void setFlags(final Collection<Flag> flags, boolean value) throws MessagingException {
-            throw new UnsupportedOperationException("POP3: No setFlags(Collection<Flag>,boolean)");
+        public void setFlags(final Set<Flag> flags, boolean value) throws MessagingException {
+            throw new UnsupportedOperationException("POP3: No setFlags(Set<Flag>,boolean)");
         }
 
         @Override
-        public void setFlags(List<? extends Message> messages, final Collection<Flag> flags, boolean value)
+        public void setFlags(List<? extends Message> messages, final Set<Flag> flags, boolean value)
         throws MessagingException {
             if (!value || !flags.contains(Flag.DELETED)) {
                 /*
@@ -1187,7 +1187,7 @@ public class Pop3Store extends Store {
         @Override
         public void setFlag(Flag flag, boolean set) throws MessagingException {
             super.setFlag(flag, set);
-            mFolder.setFlags(Collections.singletonList(this), Collections.singletonList(flag), set);
+            mFolder.setFlags(Collections.singletonList(this), Collections.singleton(flag), set);
         }
 
         @Override
diff --git a/src/com/fsck/k9/mail/store/WebDavStore.java b/src/com/fsck/k9/mail/store/WebDavStore.java
index a58a82779..27d0c7ccb 100644
--- a/src/com/fsck/k9/mail/store/WebDavStore.java
+++ b/src/com/fsck/k9/mail/store/WebDavStore.java
@@ -1781,7 +1781,7 @@ public class WebDavStore extends Store {
         }
 
         @Override
-        public void setFlags(List<? extends Message> messages, final Collection<Flag> flags, boolean value)
+        public void setFlags(List<? extends Message> messages, final Set<Flag> flags, boolean value)
         throws MessagingException {
             String[] uids = new String[messages.size()];
 
@@ -1929,9 +1929,9 @@ public class WebDavStore extends Store {
         }
 
         @Override
-        public void setFlags(final Collection<Flag> flags, boolean value) throws MessagingException {
+        public void setFlags(final Set<Flag> flags, boolean value) throws MessagingException {
             Log.e(K9.LOG_TAG,
-                  "Unimplemented method setFlags(Collection<Flag>, boolean) breaks markAllMessagesAsRead and EmptyTrash");
+                  "Unimplemented method setFlags(Set<Flag>, boolean) breaks markAllMessagesAsRead and EmptyTrash");
             // Try to make this efficient by not retrieving all of the messages
         }
     }
@@ -2029,7 +2029,7 @@ public class WebDavStore extends Store {
         @Override
         public void setFlag(Flag flag, boolean set) throws MessagingException {
             super.setFlag(flag, set);
-            mFolder.setFlags(Collections.singletonList(this), Collections.singletonList(flag), set);
+            mFolder.setFlags(Collections.singletonList(this), Collections.singleton(flag), set);
         }
     }
 
diff --git a/src/com/fsck/k9/mail/store/local/LocalFolder.java b/src/com/fsck/k9/mail/store/local/LocalFolder.java
index 9a1058099..bc5895f04 100644
--- a/src/com/fsck/k9/mail/store/local/LocalFolder.java
+++ b/src/com/fsck/k9/mail/store/local/LocalFolder.java
@@ -1724,7 +1724,7 @@ public class LocalFolder extends Folder implements Serializable {
     }
 
     @Override
-    public void setFlags(final List<? extends Message> messages, final Collection<Flag> flags, final boolean value)
+    public void setFlags(final List<? extends Message> messages, final Set<Flag> flags, final boolean value)
     throws MessagingException {
         open(OPEN_MODE_RW);
 
@@ -1752,7 +1752,7 @@ public class LocalFolder extends Folder implements Serializable {
     }
 
     @Override
-    public void setFlags(final Collection<Flag> flags, boolean value)
+    public void setFlags(final Set<Flag> flags, boolean value)
     throws MessagingException {
         open(OPEN_MODE_RW);
         for (Message message : getMessages(null)) {