diff -Nru akonadi-15.12.1/debian/changelog akonadi-15.12.3/debian/changelog
--- akonadi-15.12.1/debian/changelog	2016-04-12 14:34:08.000000000 +0000
+++ akonadi-15.12.3/debian/changelog	2016-04-14 21:44:15.000000000 +0000
@@ -1,15 +1,9 @@
-akonadi (4:15.12.1-0ubuntu4) xenial; urgency=medium
+akonadi (4:15.12.3-0ubuntu1) xenial; urgency=medium
+  
+  * New upstream bugfix release
+      * New upstream bugfix release
 
-  * Depend on mysql-{server,client}-core-5.7, not 5.6, for the switch to
-    MySQL 5.7.
-
- -- Robie Basak <robie.basak@ubuntu.com>  Tue, 12 Apr 2016 14:33:29 +0000
-
-akonadi (4:15.12.1-0ubuntu3) xenial; urgency=medium
-
-  * Rebuild against libmysqlclient20.
-
- -- Robie Basak <robie.basak@ubuntu.com>  Tue, 05 Apr 2016 12:11:54 +0000
+ -- Scarlett Clark <sgclark@kubuntu.org>  Thu, 14 Apr 2016 14:44:15 -0700
 
 akonadi (4:15.12.1-0ubuntu2) xenial; urgency=medium
 
diff -Nru akonadi-15.12.1/debian/control akonadi-15.12.3/debian/control
--- akonadi-15.12.1/debian/control	2016-04-12 14:33:29.000000000 +0000
+++ akonadi-15.12.3/debian/control	2016-04-14 21:44:15.000000000 +0000
@@ -70,8 +70,8 @@
 Section: misc
 Architecture: all
 Depends: libqt5sql5-mysql,
-         mysql-client-core-5.7 | virtual-mysql-client-core,
-         mysql-server-core-5.7 | virtual-mysql-server-core,
+         mysql-client-core-5.6 | virtual-mysql-client-core,
+         mysql-server-core-5.6 | virtual-mysql-server-core,
          ${misc:Depends}
 Recommends: akonadi-server
 Replaces: akonadi-server (<< 1.3.60~)
diff -Nru akonadi-15.12.1/src/akonadicontrol/agenttype.cpp akonadi-15.12.3/src/akonadicontrol/agenttype.cpp
--- akonadi-15.12.1/src/akonadicontrol/agenttype.cpp	2015-12-17 22:42:16.000000000 +0000
+++ akonadi-15.12.3/src/akonadicontrol/agenttype.cpp	2016-03-11 00:10:30.000000000 +0000
@@ -47,20 +47,14 @@
     if (value.isNull()) {
         return QString();
     } else if (value.canConvert<QString>()) {
-        return QString::fromUtf8(value.toByteArray());
+        return value.toString();
     } else if (value.canConvert<QStringList>()) {
         // This is a workaround for QSettings interpreting value with a comma as
         // a QStringList, which is not compatible with KConfig. KConfig reads everything
         // as a QByteArray and splits it to a list when requested. See BKO#330010
         // TODO KF5: If we end up in Tier 2 or above, depend on KConfig for parsing
         // .desktop files
-        const QStringList parts = value.toStringList();
-        QStringList utf8Parts;
-        utf8Parts.reserve(parts.size());
-        Q_FOREACH (const QString &part, parts) {
-            utf8Parts << QString::fromUtf8(part.toLatin1());
-        }
-        return utf8Parts.join(QStringLiteral(", "));
+        return value.toStringList().join(QStringLiteral(", "));
     } else {
         akError() << "Agent desktop file" << file.fileName() << "contains invalid value for key" << key;
         return QString();
@@ -72,6 +66,7 @@
     Q_UNUSED(manager);
 
     QSettings file(fileName, QSettings::IniFormat);
+    file.setIniCodec("UTF-8");
     file.beginGroup(QStringLiteral("Desktop Entry"));
 
     Q_FOREACH (const QString &key, file.allKeys()) {
diff -Nru akonadi-15.12.1/src/server/storage/dbupdate.xml akonadi-15.12.3/src/server/storage/dbupdate.xml
--- akonadi-15.12.1/src/server/storage/dbupdate.xml	2015-12-17 22:42:16.000000000 +0000
+++ akonadi-15.12.3/src/server/storage/dbupdate.xml	2016-03-11 00:10:30.000000000 +0000
@@ -256,4 +256,64 @@
                                 WHERE PimItemTable.id IS NULL)
     </raw-sql>
   </update>
+  <!--
+   Update 31 fixes missing constraints on relation tables in MySQL.
+   We cannot just let DbInitializer create the foreign keys there, because data
+   in the relation tables might reference rows from other tables that no longer
+   exist.
+  -->
+  <update version="31" abortOnFailure="false">
+    <!-- PimItemFlagRelation -->
+    <raw-sql backends="mysql">DELETE FROM PimItemFlagRelation WHERE pimItem_id IN (
+                                SELECT id FROM (
+                                SELECT pimItem_id AS id FROM PimItemFlagRelation
+                                LEFT JOIN PimItemTable ON PimItemFlagRelation.pimItem_id = PimItemTable.id
+                                WHERE PimItemTable.id IS NULL) x)
+    </raw-sql>
+    <raw-sql backends="mysql">DELETE FROM PimItemFlagRelation WHERE flag_id IN (
+                                SELECT id FROM (
+                                SELECT flag_id AS id FROM PimItemFlagRelation
+                                LEFT JOIN FlagTable ON PimItemFlagRelation.flag_id = FlagTable.id
+                                WHERE FlagTable.id IS NULL) x)
+    </raw-sql>
+    <!-- PimItemTagRelation -->
+    <raw-sql backends="mysql">DELETE FROM PimItemTagRelation WHERE pimItem_id IN (
+                                SELECT id FROM (
+                                SELECT pimItem_id AS id FROM PimItemTagRelation
+                                LEFT JOIN PimItemTable ON PimItemTagRelation.pimItem_id = PimItemTable.id
+                                WHERE PimItemTable.id IS NULL) x)
+    </raw-sql>
+    <raw-sql backends="mysql">DELETE FROM PimItemTagRelation WHERE tag_id IN (
+                                SELECT id FROM (
+                                SELECT tag_id AS id FROM PimItemTagRelation
+                                LEFT JOIN TagTable ON PimItemTagRelation.tag_id = TagTable.id
+                                WHERE TagTable.id IS NULL) x)
+    </raw-sql>
+    <!-- CollectionMimeTypeRelation -->
+    <raw-sql backends="mysql">DELETE FROM CollectionMimeTypeRelation WHERE collection_id IN (
+                                SELECT id FROM (
+                                SELECT collection_id AS id  FROM CollectionMimeTypeRelation
+                                LEFT JOIN CollectionTable ON CollectionMimeTypeRelation.collection_id = CollectionTable.id
+                                WHERE CollectionTable.id IS NULL) x)
+    </raw-sql>
+    <raw-sql backends="mysql">DELETE FROM CollectionMimeTypeRelation WHERE mimeType_id IN (
+                                SELECT id FROM (
+                                SELECT mimeType_id AS id FROM CollectionMimeTypeRelation
+                                LEFT JOIN MimeTypeTable ON CollectionMimeTypeRelation.mimeType_id = MimeTypeTable.id
+                                WHERE MimeTypeTable.id IS NULL) x)
+    </raw-sql>
+    <!-- CollectionPimItemRelation -->
+    <raw-sql backends="mysql">DELETE FROM CollectionPimItemRelation WHERE collection_id IN (
+                                SELECT id FROM (
+                                SELECT collection_id AS id FROM CollectionPimItemRelation
+                                LEFT JOIN CollectionTable ON CollectionPimItemRelation.collection_id = CollectionTable.id
+                                WHERE CollectionTable.id IS NULL) x)
+    </raw-sql>
+    <raw-sql backends="mysql">DELETE FROM CollectionPimItemRelation WHERE pimItem_id IN (
+                                SELECT id FROM (
+                                SELECT pimItem_id AS id FROM CollectionPimItemRelation
+                                LEFT JOIN PimItemTable ON CollectionPimItemRelation.pimItem_id = PimItemTable.id
+                                WHERE PimItemTable.id IS NULL) x)
+    </raw-sql>
+  </update>
 </updates>
diff -Nru akonadi-15.12.1/src/server/storage/mysql-global.conf akonadi-15.12.3/src/server/storage/mysql-global.conf
--- akonadi-15.12.1/src/server/storage/mysql-global.conf	2015-12-17 22:42:16.000000000 +0000
+++ akonadi-15.12.3/src/server/storage/mysql-global.conf	2016-03-11 00:10:30.000000000 +0000
@@ -37,9 +37,9 @@
 # use InnoDB for transactions and better crash recovery
 default_storage_engine=innodb
 
-# memory pool InnoDB uses to store data dictionary information and other internal data structures (default:1M)
-# Deprecated in MySQL >= 5.6.3
-innodb_additional_mem_pool_size=1M
+# memory pool InnoDB uses to store data dictionary information and other internal data structures (default:8M)
+# Deprecated in MySQL >= 5.6.3, removed in 5.7 (works in MariaDB)
+# innodb_additional_mem_pool_size=8M
 
 # memory buffer InnoDB uses to cache data and indexes of its tables (default:128M)
 # Larger values means less I/O