diff --git a/k9mail-library/build.gradle b/k9mail-library/build.gradle
index ad6a7fceb..b32294f08 100644
--- a/k9mail-library/build.gradle
+++ b/k9mail-library/build.gradle
@@ -20,11 +20,12 @@ android {
 
     defaultConfig {
         minSdkVersion 15
-        targetSdkVersion 17
+        targetSdkVersion 21
     }
 
     lintOptions {
-        abortOnError false
+        abortOnError true
+        warningsAsErrors true
         lintConfig file("$rootProject.projectDir/config/lint/lint.xml")
     }
 
diff --git a/k9mail-library/src/main/java/com/fsck/k9/mail/store/pop3/Pop3Store.java b/k9mail-library/src/main/java/com/fsck/k9/mail/store/pop3/Pop3Store.java
index d1d096427..922115e4e 100644
--- a/k9mail-library/src/main/java/com/fsck/k9/mail/store/pop3/Pop3Store.java
+++ b/k9mail-library/src/main/java/com/fsck/k9/mail/store/pop3/Pop3Store.java
@@ -1,6 +1,7 @@
 
 package com.fsck.k9.mail.store.pop3;
 
+import android.annotation.SuppressLint;
 import android.util.Log;
 
 import com.fsck.k9.mail.*;
@@ -278,6 +279,7 @@ public class Pop3Store extends RemoteStore {
         private InputStream mIn;
         private OutputStream mOut;
         private Map<String, Pop3Message> mUidToMsgMap = new HashMap<String, Pop3Message>();
+        @SuppressLint("UseSparseArrays")
         private Map<Integer, Pop3Message> mMsgNumToMsgMap = new HashMap<Integer, Pop3Message>();
         private Map<String, Integer> mUidToMsgNumMap = new HashMap<String, Integer>();
         private String mName;
diff --git a/k9mail-library/src/main/java/com/fsck/k9/mail/store/webdav/WebDavSocketFactory.java b/k9mail-library/src/main/java/com/fsck/k9/mail/store/webdav/WebDavSocketFactory.java
index 91c43a0a3..3beeb0131 100644
--- a/k9mail-library/src/main/java/com/fsck/k9/mail/store/webdav/WebDavSocketFactory.java
+++ b/k9mail-library/src/main/java/com/fsck/k9/mail/store/webdav/WebDavSocketFactory.java
@@ -9,10 +9,8 @@ import com.fsck.k9.mail.ssl.TrustManagerFactory;
 import java.io.IOException;
 import java.net.InetAddress;
 import java.net.Socket;
-import java.net.UnknownHostException;
 import java.security.KeyManagementException;
 import java.security.NoSuchAlgorithmException;
-import java.security.SecureRandom;
 
 import javax.net.ssl.SSLContext;
 import javax.net.ssl.SSLSocket;
@@ -32,7 +30,7 @@ public class WebDavSocketFactory implements LayeredSocketFactory {
         SSLContext sslContext = SSLContext.getInstance("TLS");
         sslContext.init(null, new TrustManager[] {
                 TrustManagerFactory.get(host, port)
-        }, new SecureRandom());
+        }, null);
         mSocketFactory = sslContext.getSocketFactory();
         mSchemeSocketFactory = org.apache.http.conn.ssl.SSLSocketFactory.getSocketFactory();
         mSchemeSocketFactory.setHostnameVerifier(
@@ -41,7 +39,7 @@ public class WebDavSocketFactory implements LayeredSocketFactory {
 
     public Socket connectSocket(Socket sock, String host, int port,
             InetAddress localAddress, int localPort, HttpParams params)
-            throws IOException, UnknownHostException, ConnectTimeoutException {
+            throws IOException, ConnectTimeoutException {
         return mSchemeSocketFactory.connectSocket(sock, host, port, localAddress, localPort, params);
     }
 
@@ -57,7 +55,7 @@ public class WebDavSocketFactory implements LayeredSocketFactory {
             final String host,
             final int port,
             final boolean autoClose
-    ) throws IOException, UnknownHostException {
+    ) throws IOException {
         SSLSocket sslSocket = (SSLSocket) mSocketFactory.createSocket(
                 socket,
                 host,