diff --git a/mailbox/elasticsearch/src/main/java/org/apache/james/mailbox/elasticsearch/json/IndexableMessage.java b/mailbox/elasticsearch/src/main/java/org/apache/james/mailbox/elasticsearch/json/IndexableMessage.java
index d36b1ec..ec58c3c 100644
--- a/mailbox/elasticsearch/src/main/java/org/apache/james/mailbox/elasticsearch/json/IndexableMessage.java
+++ b/mailbox/elasticsearch/src/main/java/org/apache/james/mailbox/elasticsearch/json/IndexableMessage.java
@@ -25,7 +25,6 @@
 import java.time.ZonedDateTime;
 import java.util.List;
 import java.util.Optional;
-import java.util.Set;
 import java.util.stream.Collectors;
 
 import org.apache.james.mailbox.MailboxSession.User;
@@ -67,7 +66,7 @@
 
     private void copyHeaderFields(HeaderCollection headerCollection, ZonedDateTime internalDate) {
         this.headers = headerCollection.getHeaders();
-        this.subjects = headerCollection.getSubjectSet();
+        this.subjects = Subjects.from(headerCollection.getSubjectSet());
         this.from = EMailers.from(headerCollection.getFromAddressSet());
         this.to = EMailers.from(headerCollection.getToAddressSet());
         this.replyTo = EMailers.from(headerCollection.getReplyToAddressSet());
@@ -124,7 +123,7 @@
     private EMailers cc;
     private EMailers bcc;
     private EMailers replyTo;
-    private Set<String> subjects;
+    private Subjects subjects;
     private String sentDate;
     private List<Property> properties;
     private List<MimePart> attachments;
@@ -211,7 +210,7 @@
     }
 
     @JsonProperty(JsonMessageConstants.SUBJECT)
-    public Set<String> getSubjects() {
+    public Subjects getSubjects() {
         return subjects;
     }
 
diff --git a/mailbox/elasticsearch/src/main/java/org/apache/james/mailbox/elasticsearch/json/Subjects.java b/mailbox/elasticsearch/src/main/java/org/apache/james/mailbox/elasticsearch/json/Subjects.java
new file mode 100644
index 0000000..214d312
--- /dev/null
+++ b/mailbox/elasticsearch/src/main/java/org/apache/james/mailbox/elasticsearch/json/Subjects.java
@@ -0,0 +1,50 @@
+/****************************************************************
+ * Licensed to the Apache Software Foundation (ASF) under one   *
+ * or more contributor license agreements.  See the NOTICE file *
+ * distributed with this work for additional information        *
+ * regarding copyright ownership.  The ASF licenses this file   *
+ * to you under the Apache License, Version 2.0 (the            *
+ * "License"); you may not use this file except in compliance   *
+ * with the License.  You may obtain a copy of the License at   *
+ *                                                              *
+ *   http://www.apache.org/licenses/LICENSE-2.0                 *
+ *                                                              *
+ * Unless required by applicable law or agreed to in writing,   *
+ * software distributed under the License is distributed on an  *
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY       *
+ * KIND, either express or implied.  See the License for the    *
+ * specific language governing permissions and limitations      *
+ * under the License.                                           *
+ ****************************************************************/
+
+package org.apache.james.mailbox.elasticsearch.json;
+
+import java.util.Set;
+
+import com.fasterxml.jackson.annotation.JsonValue;
+import com.google.common.base.Joiner;
+import com.google.common.base.Preconditions;
+
+public class Subjects implements Serializable {
+
+    public static Subjects from(Set<String> subjects) {
+        Preconditions.checkNotNull(subjects, "'subjects' is mandatory");
+        return new Subjects(subjects);
+    }
+
+    private final Set<String> subjects;
+
+    private Subjects(Set<String> subjects) {
+        this.subjects = subjects;
+    }
+
+    @JsonValue
+    public Set<String> getSubjects() {
+        return subjects;
+    }
+
+    @Override
+    public String serialize() {
+        return Joiner.on(" ").join(subjects);
+    }
+}
diff --git a/mailbox/elasticsearch/src/test/java/org/apache/james/mailbox/elasticsearch/json/SubjectsTest.java b/mailbox/elasticsearch/src/test/java/org/apache/james/mailbox/elasticsearch/json/SubjectsTest.java
new file mode 100644
index 0000000..a4398d1
--- /dev/null
+++ b/mailbox/elasticsearch/src/test/java/org/apache/james/mailbox/elasticsearch/json/SubjectsTest.java
@@ -0,0 +1,66 @@
+/****************************************************************
+ * Licensed to the Apache Software Foundation (ASF) under one   *
+ * or more contributor license agreements.  See the NOTICE file *
+ * distributed with this work for additional information        *
+ * regarding copyright ownership.  The ASF licenses this file   *
+ * to you under the Apache License, Version 2.0 (the            *
+ * "License"); you may not use this file except in compliance   *
+ * with the License.  You may obtain a copy of the License at   *
+ *                                                              *
+ *   http://www.apache.org/licenses/LICENSE-2.0                 *
+ *                                                              *
+ * Unless required by applicable law or agreed to in writing,   *
+ * software distributed under the License is distributed on an  *
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY       *
+ * KIND, either express or implied.  See the License for the    *
+ * specific language governing permissions and limitations      *
+ * under the License.                                           *
+ ****************************************************************/
+
+package org.apache.james.mailbox.elasticsearch.json;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import org.junit.Test;
+
+import com.google.common.base.Joiner;
+import com.google.common.collect.ImmutableSet;
+
+public class SubjectsTest {
+
+    @Test
+    public void fromShouldThrowWhenSetIsNull() {
+        assertThatThrownBy(() -> Subjects.from(null))
+            .isInstanceOf(NullPointerException.class)
+            .hasMessage("'subjects' is mandatory");
+    }
+
+    @Test
+    public void serializeShouldReturnEmptyWhenEmptySet() {
+        Subjects subjects = Subjects.from(ImmutableSet.of());
+
+        assertThat(subjects.serialize()).isEmpty();
+    }
+
+    @Test
+    public void serializeShouldNotJoinWhenOneElement() {
+        String expected = "subject";
+        Subjects subjects = Subjects.from(ImmutableSet.of(expected));
+
+        assertThat(subjects.serialize()).isEqualTo(expected);
+    }
+
+    @Test
+    public void serializeShouldJoinWhenMultipleElements() {
+        String subject = "subject";
+        String subject2 = "subject2";
+        String subject3 = "subject3";
+
+        String expected = Joiner.on(" ").join(subject, subject2, subject3);
+
+        Subjects subjects = Subjects.from(ImmutableSet.of(subject, subject2, subject3));
+
+        assertThat(subjects.serialize()).isEqualTo(expected);
+    }
+}