diff --git a/k9mail-library/src/main/java/com/fsck/k9/mail/internet/DecoderUtil.java b/k9mail-library/src/main/java/com/fsck/k9/mail/internet/DecoderUtil.java
index 548f2dc08..6fc2e8a8a 100644
--- a/k9mail-library/src/main/java/com/fsck/k9/mail/internet/DecoderUtil.java
+++ b/k9mail-library/src/main/java/com/fsck/k9/mail/internet/DecoderUtil.java
@@ -21,6 +21,13 @@ import timber.log.Timber;
  * it has to be determined with the sender address, the mailer and so on.
  */
 class DecoderUtil {
+
+    private static class EncodedWord {
+        private String charset;
+        private String encoding;
+        private String encodedText;
+    }
+
     /**
      * Decodes an encoded word encoded with the 'B' encoding (described in
      * RFC 2047) found in a header field body.
@@ -48,7 +55,7 @@ class DecoderUtil {
      * @param charset the Java charset to use.
      * @return the decoded string.
      */
-    private static String decodeQ(String encodedWord, String charset) {
+    static String decodeQ(String encodedWord, String charset) {
 
         /*
          * Replace _ with =20
@@ -93,15 +100,15 @@ class DecoderUtil {
             return body;
         }
 
+        EncodedWord previousWord = null;
         int previousEnd = 0;
-        boolean previousWasEncoded = false;
 
         StringBuilder sb = new StringBuilder();
 
         while (true) {
             int begin = body.indexOf("=?", previousEnd);
             if (begin == -1) {
-                sb.append(body.substring(previousEnd));
+                decodePreviousAndAppendSuffix(sb, previousWord, body, previousEnd);
                 return sb.toString();
             }
 
@@ -110,43 +117,77 @@ class DecoderUtil {
             // to find the two '?' in the "header", before looking for the final "?=".
             int qm1 = body.indexOf('?', begin + 2);
             if (qm1 == -1) {
-                sb.append(body.substring(previousEnd));
+                decodePreviousAndAppendSuffix(sb, previousWord, body, previousEnd);
                 return sb.toString();
             }
 
             int qm2 = body.indexOf('?', qm1 + 1);
             if (qm2 == -1) {
-                sb.append(body.substring(previousEnd));
+                decodePreviousAndAppendSuffix(sb, previousWord, body, previousEnd);
                 return sb.toString();
             }
 
             int end = body.indexOf("?=", qm2 + 1);
             if (end == -1) {
-                sb.append(body.substring(previousEnd));
+                decodePreviousAndAppendSuffix(sb, previousWord, body, previousEnd);
                 return sb.toString();
             }
             end += 2;
 
             String sep = body.substring(previousEnd, begin);
 
-            String decoded = decodeEncodedWord(body, begin, end, message);
-            if (decoded == null) {
+            EncodedWord word = extractEncodedWord(body, begin, end, message);
+
+            if (previousWord == null) {
                 sb.append(sep);
-                sb.append(body.substring(begin, end));
-            } else {
-                if (!previousWasEncoded || !CharsetUtil.isWhitespace(sep)) {
-                    sb.append(sep);
+                if (word == null) {
+                    sb.append(body.substring(begin, end));
+                }
+            } else {
+                if (word == null) {
+                    sb.append(decodeEncodedWord(previousWord));
+                    sb.append(sep);
+                    sb.append(body.substring(begin, end));
+                } else {
+                    if (!CharsetUtil.isWhitespace(sep)) {
+                        sb.append(decodeEncodedWord(previousWord));
+                        sb.append(sep);
+                    } else if (previousWord.encoding.equals(word.encoding) &&
+                            previousWord.charset.equals(word.charset)) {
+                        word.encodedText = previousWord.encodedText + word.encodedText;
+                    } else {
+                        sb.append(decodeEncodedWord(previousWord));
+                    }
                 }
-                sb.append(decoded);
             }
 
+            previousWord = word;
             previousEnd = end;
-            previousWasEncoded = decoded != null;
         }
     }
 
-    // return null on error
-    private static String decodeEncodedWord(String body, int begin, int end, Message message) {
+    private static void decodePreviousAndAppendSuffix(StringBuilder sb, EncodedWord previousWord, String body,
+            int previousEnd) {
+
+        if (previousWord != null) {
+            sb.append(decodeEncodedWord(previousWord));
+        }
+
+        sb.append(body.substring(previousEnd));
+    }
+
+    private static String decodeEncodedWord(EncodedWord word) {
+        if (word.encoding.equals("Q")) {
+            return decodeQ(word.encodedText, word.charset);
+        } else if (word.encoding.equals("B")) {
+            return DecoderUtil.decodeB(word.encodedText, word.charset);
+        } else {
+            Timber.w("Warning: Unknown encoding '%s'", word.encoding);
+            return null;
+        }
+    }
+
+    private static EncodedWord extractEncodedWord(String body, int begin, int end, Message message) {
         int qm1 = body.indexOf('?', begin + 2);
         if (qm1 == end - 2)
             return null;
@@ -171,13 +212,17 @@ class DecoderUtil {
             return null;
         }
 
+        EncodedWord encodedWord = new EncodedWord();
+        encodedWord.charset = charset;
         if (encoding.equalsIgnoreCase("Q")) {
-            return decodeQ(encodedText, charset);
+            encodedWord.encoding = "Q";
         } else if (encoding.equalsIgnoreCase("B")) {
-            return DecoderUtil.decodeB(encodedText, charset);
+            encodedWord.encoding = "B";
         } else {
             Timber.w("Warning: Unknown encoding in encoded word '%s'", body.substring(begin, end));
             return null;
         }
+        encodedWord.encodedText = encodedText;
+        return encodedWord;
     }
 }
diff --git a/k9mail-library/src/test/java/com/fsck/k9/mail/internet/DecoderUtilTest.java b/k9mail-library/src/test/java/com/fsck/k9/mail/internet/DecoderUtilTest.java
index cd693c3be..a0c205e7e 100644
--- a/k9mail-library/src/test/java/com/fsck/k9/mail/internet/DecoderUtilTest.java
+++ b/k9mail-library/src/test/java/com/fsck/k9/mail/internet/DecoderUtilTest.java
@@ -11,117 +11,207 @@ import static org.junit.Assert.assertEquals;
 
 @RunWith(K9LibRobolectricTestRunner.class)
 public class DecoderUtilTest {
+    private static final String INVALID = "=?utf-8?Q??=";
+
 
     @Test
-    public void testDecodeEncodedWords() {
-        String body, expect;
-        MimeMessage message;
+    public void decodeEncodedWords_withInvalidEncodedWord_shouldReturnInputText() {
+        // We use INVALID as instance of an invalid encoded word in tests. If at some point we decide to change the code
+        // to recognize empty encoded text as valid and decode it to an empty string, a lot of tests will break.
+        // Hopefully this test will help the developer figure out why the other tests broke.
+        assertInputDecodesToExpected(INVALID, INVALID);
+    }
 
-        body = "abc";
-        expect = "abc";
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_with_unencoded_data_returns_original_text() {
+        assertInputDecodesToExpected("abc", "abc");
+    }
 
-        body = "=?us-ascii?q?abc?=";
-        expect = "abc";
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_withAsciiCharset_encoded_data_returns_text() {
+        assertInputDecodesToExpected("=?us-ascii?q?abc?=", "abc");
+    }
 
-        body = "=?";
-        expect = "=?";
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_withStartOnly_encoding_format_returnAsText() {
+        assertInputDecodesToExpected("=?", "=?");
+    }
 
-        body = "=??";
-        expect = "=??";
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_withEncodedWordAndOnlyStartOfEncodedWord_shouldDecodeAndAddSuffix() {
+        assertInputDecodesToExpected("=?utf-8?Q?abc?= =?", "abc =?");
+    }
 
-        body = "=???";
-        expect = "=???";
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_withStartAndSeparatorOnly_returnAsText() {
+        assertInputDecodesToExpected("=??", "=??");
+    }
 
-        body = "=????";
-        expect = "=????";
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_withEncodedWordAndOnlyStartAndSeparatorOfEncodedWord_shouldDecodeAndAddSuffix() {
+        assertInputDecodesToExpected("=?utf-8?Q?abc?= =??", "abc =??");
+    }
 
-        body = "=????=";
-        expect = "=????=";
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_withStartAnd2SeparatorOnly_returnAsText() {
+        assertInputDecodesToExpected("=???", "=???");
+    }
 
-        body = "=??q??=";
-        expect = "=??q??=";
-        ;
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_withEncodedWordAndOnlyStartAndTwoSeparatorsOfEncodedWord_shouldDecodeAndAddSuffix() {
+        assertInputDecodesToExpected("=?utf-8?Q?abc?= =???", "abc =???");
+    }
 
-        body = "=??q?a?=";
-        expect = "a";
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_withStartAnd3SeparatorOnly_returnAsText() {
+        assertInputDecodesToExpected("=????", "=????");
+    }
 
-        body = "=??=";
-        expect = "=??=";
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_withEncodedWordAndOnlyStartAndThreeSeparatorsOfEncodedWord_shouldDecodeAndAddSuffix() {
+        assertInputDecodesToExpected("=?utf-8?Q?abc?= =????", "abc =????");
+    }
 
-        body = "=?x?=";
-        expect = "=?x?=";
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_withSeparatorsOnly_returnAsText() {
+        assertInputDecodesToExpected("=????=", "=????=");
+    }
 
-        body = "=?x??=";
-        expect = "=?x??=";
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_withMissingCharset_returnAsText() {
+        assertInputDecodesToExpected("=??q??=", "=??q??=");
+    }
 
-        body = "=?x?q?=";
-        expect = "=?x?q?=";
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_withTextAndMissingCharset_returnAsText() {
+        assertInputDecodesToExpected("=??q?a?=", "a");
+    }
 
-        body = "=?x?q??=";
-        expect = "=?x?q??=";
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_withNoTextCharsetOrEncoding_returnAsText() {
+        assertInputDecodesToExpected("=??=", "=??=");
+    }
 
-        body = "=?x?q?X?=";
-        expect = "X";
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_with_MissingEncodingAndData_returnAsText() {
+        assertInputDecodesToExpected("=?x?=", "=?x?=");
+    }
 
-        // invalid base64 string
-        body = "=?us-ascii?b?abc?=";
-        expect = "";
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_withMissingEncoding_returnAsText() {
+        assertInputDecodesToExpected("=?x??=", "=?x??=");
+    }
 
-        // broken encoded header
-        body = "=?us-ascii?q?abc?= =?";
-        expect = "abc =?";
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_with_incompleteEncodingFormat_returnAsText() {
+        assertInputDecodesToExpected("=?x?q?=", "=?x?q?=");
+    }
 
-        body = "=?x?= =?";
-        expect = "=?x?= =?";
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_with_unrecognisedEncoding_withEmptyEncodedData_returnAsText() {
+        assertInputDecodesToExpected("=?x?q??=", "=?x?q??=");
+    }
 
-        //Multi encoded header
-        body = "=?utf-8?B?5Liq5Lq66YKu566xOkJVRyAjMzAyNDY6OumCruS7tuato+aWh+mZhOS7tuWQ?=\n" +
-                "=?utf-8?B?jeensOecgeeVpeaYvuekuuS8mOWMlg==?=";
-        expect = "个人邮箱:BUG #30246::邮件正文附件��称省略显示优化";
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_withUnrecognisedEncoding_withEncodedData_return_encoded_data() {
+        assertInputDecodesToExpected("=?x?q?X?=", "X");
+    }
 
-        //Non utf-8 encoding
-        body = "=?gb2312?B?Obv9t9az6cnu29rHsLqju6rHyLPHSlfN8rrAvsa16qOsuPzT0DIwvNIzOTnU?= " +
-                "=?gb2312?B?qr6r0aG439DHytTLr77Gteq1yMTjwLSjoaOoQUSjqQ?=";
-        expect = "9积分抽深圳前海华侨城JW万豪酒店，更有20家399��精选高星试睡酒店等你来！（AD�";
-        message = null;
-        assertEquals(expect, DecoderUtil.decodeEncodedWords(body, message));
+    @Test
+    public void decodeEncodedWords_withInvalidBase64String_returnsEmptyString() {
+        assertInputDecodesToExpected("=?us-ascii?b?abc?=", "");
+    }
+
+    @Test
+    public void decodeEncodedWords_withPartiallyEncoded_returnsBothSections() {
+        assertInputDecodesToExpected("=?us-ascii?q?abc?= =?", "abc =?");
+    }
+
+    @Test
+    public void decodeEncodedWords_withPartiallyEncodedAfter_returnsBothSections() {
+        assertInputDecodesToExpected("def=?us-ascii?q?abc?=", "defabc");
+    }
+
+    @Test
+    public void decodeEncodedWords_withUnrecognisedCharset_returnsEncodedData() {
+        assertInputDecodesToExpected("=?x?= =?", "=?x?= =?");
+    }
+
+    @Test
+    public void decodeEncodedWords_withMultipleEncodedSections_decodesBoth() {
+        assertInputDecodesToExpected("=?us-ascii?q?abc?= =?us-ascii?q?def?=", "abcdef");
+    }
+
+    @Test
+    public void decodeEncodedWords_withMultipleEncodedSections_decodesSequentialSectionTogether() {
+        // Splitting mid-character is RFC2047 non-compliant but seen in practice.
+        // "=?utf-8?B?b2hhaSDw?=" individually decodes to "ohai �"
+        // "=?utf-8?B?n5Kp==?=" individually decodes to "���"
+        // (invalid bytes in a UTF-8 sequence are replaced with the replacement character)
+        assertInputDecodesToExpected("=?utf-8?B?b2hhaSDw?= =?utf-8?B?n5Kp?=", "ohai 💩");
+    }
+
+    @Test
+    public void decodeEncodedWords_withMultipleEncodedSectionsButCharsetAndEncodingDifferingInCase_decodesSequentialSectionTogether() {
+        assertInputDecodesToExpected("=?utf-8?B?b2hhaSDw?= =?UTF-8?b?n5Kp?=", "ohai 💩");
+    }
+
+    @Test
+    public void decodeEncodedWords_withEncodedWordWhitespaceInvalidEncodedWord_shouldOnlyDecodeEncodedWord() {
+        assertInputDecodesToExpected("=?utf-8?Q?abc?=   " + INVALID, "abc   " + INVALID);
+    }
+
+    @Test
+    public void decodeEncodedWords_withInvalidEncodedWordWhitespaceInvalidEncodedWord_shouldReturnInputText() {
+        String input = INVALID + "   " + INVALID;
+        assertInputDecodesToExpected(input, input);
+    }
+
+    @Test
+    public void decodeEncodedWords_withEncodedWordNonWhitespaceSeparatorEncodedWord_shouldDecodeBothAndKeepSeparator() {
+        assertInputDecodesToExpected("=?utf-8?Q?ab?= -- =?utf-8?Q?cd?=", "ab -- cd");
+    }
+
+    @Test
+    public void decodeEncodedWords_withInvalidEncodedWordWhitespaceEncodedWord_shouldOnlyDecodeEncodedWord() {
+        assertInputDecodesToExpected(INVALID + "   =?utf-8?Q?abc?=", INVALID + "   abc");
+    }
+
+    @Test
+    public void decodeEncodedWords_withEncodedWordFollowedByEncodedWordWithDifferentEncoding_shouldDecodeIndividually() {
+        assertInputDecodesToExpected("=?utf-8?Q?ab?= =?utf-8?B?Y2Q=?=", "abcd");
+    }
+
+    @Test
+    public void decodeEncodedWords_withEncodedWordSeparatorEncodedWordWithDifferentEncoding_shouldDecodeIndividuallyAndKeepSeparator() {
+        assertInputDecodesToExpected("=?utf-8?Q?ab?= / =?utf-8?B?Y2Q=?=", "ab / cd");
+    }
+
+    @Test
+    public void decodeEncodedWords_withEncodedWordFollowedByEncodedWordWithDifferentCharset_shouldDecodeIndividually() {
+        assertInputDecodesToExpected("=?us-ascii?Q?oh_no_?= =?utf-8?Q?=F0=9F=92=A9?=", "oh no 💩");
+    }
+
+    @Test
+    public void decodeEncodedWords_withRFC2047examples_decodesCorrectly() {
+        assertInputDecodesToExpected("(=?ISO-8859-1?Q?a?=)", "(a)");
+
+        assertInputDecodesToExpected("(=?ISO-8859-1?Q?a?= b)", "(a b)");
+
+        assertInputDecodesToExpected("(=?ISO-8859-1?Q?a?= =?ISO-8859-1?Q?b?=)", "(ab)");
+
+        assertInputDecodesToExpected("(=?ISO-8859-1?Q?a?=  =?ISO-8859-1?Q?b?=)", "(ab)");
+
+        assertInputDecodesToExpected("(=?ISO-8859-1?Q?a?=     \n    =?ISO-8859-1?Q?b?=)", "(ab)");
+
+        assertInputDecodesToExpected("(=?ISO-8859-1?Q?a_b?=)", "(a b)");
+
+        assertInputDecodesToExpected("(=?ISO-8859-1?Q?a?= =?ISO-8859-2?Q?_b?=)", "(a b)");
+    }
+
+
+    private void assertInputDecodesToExpected(String input, String expected) {
+        String decodedText = DecoderUtil.decodeEncodedWords(input, null);
+        assertEquals(expected, decodedText);
     }
 }