diff --git a/smack-core/src/main/java/org/jivesoftware/smack/AbstractXMPPConnection.java b/smack-core/src/main/java/org/jivesoftware/smack/AbstractXMPPConnection.java
index fe82c07f6..d65fb0b24 100644
--- a/smack-core/src/main/java/org/jivesoftware/smack/AbstractXMPPConnection.java
+++ b/smack-core/src/main/java/org/jivesoftware/smack/AbstractXMPPConnection.java
@@ -294,6 +294,12 @@ public abstract class AbstractXMPPConnection implements XMPPConnection {
             Thread thread = new Thread(runnable);
             thread.setName("Smack Cached Executor");
             thread.setDaemon(true);
+            thread.setUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {
+                @Override
+                public void uncaughtException(Thread t, Throwable e) {
+                    LOGGER.log(Level.WARNING, t + " encountered uncaught exception", e);
+                }
+            });
             return thread;
         }
     });
diff --git a/smack-core/src/main/java/org/jivesoftware/smack/ConnectionConfiguration.java b/smack-core/src/main/java/org/jivesoftware/smack/ConnectionConfiguration.java
index 7115a2bc3..d14a82d46 100644
--- a/smack-core/src/main/java/org/jivesoftware/smack/ConnectionConfiguration.java
+++ b/smack-core/src/main/java/org/jivesoftware/smack/ConnectionConfiguration.java
@@ -1,6 +1,6 @@
 /**
  *
- * Copyright 2003-2007 Jive Software, 2017-2018 Florian Schmaus.
+ * Copyright 2003-2007 Jive Software, 2017-2019 Florian Schmaus.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,6 +25,8 @@ import java.util.Collection;
 import java.util.Collections;
 import java.util.HashSet;
 import java.util.Set;
+import java.util.logging.Level;
+import java.util.logging.Logger;
 
 import javax.net.SocketFactory;
 import javax.net.ssl.HostnameVerifier;
@@ -46,6 +48,7 @@ import org.jxmpp.jid.impl.JidCreate;
 import org.jxmpp.jid.parts.Resourcepart;
 import org.jxmpp.stringprep.XmppStringprepException;
 import org.minidns.dnsname.DnsName;
+import org.minidns.dnsname.InvalidDnsNameException;
 import org.minidns.util.InetAddressUtil;
 
 /**
@@ -61,6 +64,8 @@ public abstract class ConnectionConfiguration {
         SmackConfiguration.getVersion();
     }
 
+    private static final Logger LOGGER = Logger.getLogger(ConnectionConfiguration.class.getName());
+
     /**
      * The XMPP domain of the XMPP Service. Usually servers use the same service name as the name
      * of the server. However, there are some servers like google where host would be
@@ -68,6 +73,8 @@ public abstract class ConnectionConfiguration {
      */
     protected final DomainBareJid xmppServiceDomain;
 
+    protected final DnsName xmppServiceDomainDnsName;
+
     protected final InetAddress hostAddress;
     protected final DnsName host;
     protected final int port;
@@ -140,6 +147,19 @@ public abstract class ConnectionConfiguration {
         if (xmppServiceDomain == null) {
             throw new IllegalArgumentException("Must define the XMPP domain");
         }
+
+        DnsName xmppServiceDomainDnsName;
+        try {
+            xmppServiceDomainDnsName = DnsName.from(xmppServiceDomain);
+        } catch (InvalidDnsNameException e) {
+            LOGGER.log(Level.INFO,
+                            "Could not transform XMPP service domain '" + xmppServiceDomain
+                          + "' to a DNS name. TLS X.509 certificate validiation may not be possible.",
+                            e);
+            xmppServiceDomainDnsName = null;
+        }
+        this.xmppServiceDomainDnsName = xmppServiceDomainDnsName;
+
         hostAddress = builder.hostAddress;
         host = builder.host;
         port = builder.port;
@@ -201,6 +221,17 @@ public abstract class ConnectionConfiguration {
         return xmppServiceDomain;
     }
 
+    /**
+     * Returns the XMPP service domain as DNS name if possible. Note that since not every XMPP address domainpart is a
+     * valid DNS name, this method may return <code>null</code>.
+     *
+     * @return the XMPP service domain as DNS name or <code>null</code>.
+     * @since 4.3.4
+     */
+    public DnsName getXmppServiceDomainAsDnsNameIfPossible() {
+        return xmppServiceDomainDnsName;
+    }
+
     /**
      * Returns the TLS security mode used when making the connection. By default,
      * the mode is {@link SecurityMode#ifpossible}.
diff --git a/smack-extensions/src/main/java/org/jivesoftware/smackx/xhtmlim/XHTMLText.java b/smack-extensions/src/main/java/org/jivesoftware/smackx/xhtmlim/XHTMLText.java
index b08507ad0..2ef66949f 100644
--- a/smack-extensions/src/main/java/org/jivesoftware/smackx/xhtmlim/XHTMLText.java
+++ b/smack-extensions/src/main/java/org/jivesoftware/smackx/xhtmlim/XHTMLText.java
@@ -105,7 +105,7 @@ public class XHTMLText {
     private XHTMLText appendOpenBodyTag(String style, String lang) {
         text.halfOpenElement(Message.BODY);
         text.xmlnsAttribute(NAMESPACE);
-        text.optElement(STYLE, style);
+        text.optAttribute(STYLE, style);
         text.xmllangAttribute(lang);
         text.rightAngleBracket();
         return this;
diff --git a/smack-tcp/src/main/java/org/jivesoftware/smack/tcp/XMPPTCPConnection.java b/smack-tcp/src/main/java/org/jivesoftware/smack/tcp/XMPPTCPConnection.java
index a08221ddb..0329ef4b1 100644
--- a/smack-tcp/src/main/java/org/jivesoftware/smack/tcp/XMPPTCPConnection.java
+++ b/smack-tcp/src/main/java/org/jivesoftware/smack/tcp/XMPPTCPConnection.java
@@ -141,6 +141,7 @@ import org.jxmpp.jid.impl.JidCreate;
 import org.jxmpp.jid.parts.Resourcepart;
 import org.jxmpp.stringprep.XmppStringprepException;
 import org.jxmpp.util.XmppStringUtils;
+import org.minidns.dnsname.DnsName;
 import org.xmlpull.v1.XmlPullParser;
 import org.xmlpull.v1.XmlPullParserException;
 
@@ -861,8 +862,27 @@ public class XMPPTCPConnection extends AbstractXMPPConnection {
         final HostnameVerifier verifier = getConfiguration().getHostnameVerifier();
         if (verifier == null) {
                 throw new IllegalStateException("No HostnameVerifier set. Use connectionConfiguration.setHostnameVerifier() to configure.");
-        } else if (!verifier.verify(getXMPPServiceDomain().toString(), sslSocket.getSession())) {
-            throw new CertificateException("Hostname verification of certificate failed. Certificate does not authenticate " + getXMPPServiceDomain());
+        }
+
+        final String verifierHostname;
+        {
+            DnsName xmppServiceDomainDnsName = getConfiguration().getXmppServiceDomainAsDnsNameIfPossible();
+            // Try to convert the XMPP service domain, which potentially includes Unicode characters, into ASCII
+            // Compatible Encoding (ACE) to match RFC3280 dNSname IA5String constraint.
+            // See also: https://bugzilla.mozilla.org/show_bug.cgi?id=280839#c1
+            if (xmppServiceDomainDnsName != null) {
+                verifierHostname = xmppServiceDomainDnsName.ace;
+            }
+            else {
+                LOGGER.log(Level.WARNING, "XMPP service domain name '" + getXMPPServiceDomain()
+                                + "' can not be represented as DNS name. TLS X.509 certificate validiation may fail.");
+                verifierHostname = getXMPPServiceDomain().toString();
+            }
+        }
+        if (!verifier.verify(verifierHostname, sslSocket.getSession())) {
+            throw new CertificateException(
+                            "Hostname verification of certificate failed. Certificate does not authenticate "
+                                            + getXMPPServiceDomain());
         }
 
         // Set that TLS was successful