diff --git a/includes/simplenews.mail.inc b/includes/simplenews.mail.inc
index 00a1f13..6c4af5b 100644
--- a/includes/simplenews.mail.inc
+++ b/includes/simplenews.mail.inc
@@ -274,7 +274,7 @@ function simplenews_mail_spool($nid = NULL, $vid = NULL, $limit = NULL) {
   // Send pending messages from database cache
   // A limited number of mails is retrieved from the spool
   $limit = isset($limit) ? $limit : variable_get('simplenews_throttle', 20);
-  if ($messages = simplenews_get_spool(SIMPLENEWS_SPOOL_PENDING, $nid, $vid, $limit)) {
+  if ($messages = simplenews_get_spool(array(SIMPLENEWS_SPOOL_PENDING, SIMPLENEWS_SPOOL_IN_PROGRESS), $limit)) {
     $count_fail = $count_success = 0;
 
     // switch to anonymous user - needed in foreground spool sends, adopted from D7 drupal_cron_run().
@@ -364,6 +364,17 @@ function simplenews_save_spool($message) {
     $status, $time);
 }
 
+/*
+ * Returns the expiration time for IN_PROGRESS status.
+ *
+ * @return int Time the message allocation expires and is resent
+ */
+function simplenews_get_expiration_time() {
+  $timeout = variable_get('simplenews_spool_progress_expiration', 3600);
+  $expiration_time = time() - $timeout;
+  return $expiration_time;
+}
+
 /**
  * Retrieve data from mail spool
  *
@@ -381,22 +392,71 @@ function simplenews_save_spool($message) {
  *  $message['status']
  *  $message['time']
  */
-function simplenews_get_spool($status, $nid = NULL, $vid = NULL, $limit = 999999) {
+
+/**
+ * This function allocates messages to be sent in current run.
+ *
+ * Drupal acquire_lock quarantees that no concurrency issue happend.
+ * If message status is SIMPLENEWS_SPOOL_IN_PROGRESS but the maximum send time has expired the message id will be returned as a message which is not allocated to another process
+ * MessageIDs to be sent in current run are returned.
+ *
+ * @param string  $status Status of data to be retrieved (0 = hold, 1 = pending, 2 = send, 3 = in progress)
+ * @param integer $limit The maximum number of mails to load from the spool
+ *
+ * @return array array with msids
+ */
+function simplenews_get_spool($status, $limit = NULL) {
   $messages = array();
+  $clauses = array();
+  $params = array();
+
+  if (!is_array($status)) {
+    $status = array($status);
+  }
+
+  foreach ($status as $s) {
+    if ($s == SIMPLENEWS_SPOOL_IN_PROGRESS) {
+      // Select messages which are allocated by another process, but where the maximum send time has expired.
+      $clauses[] = '(s.status = %d AND s.timestamp < %d)';
+      $params[] = $s;
+      $params[] = simplenews_get_expiration_time();
+    }
+    else {
+      $clauses[] = 's.status = %d';
+      $params[] = $s;
+    }
+  }
 
-  $result = db_query_range("
-    SELECT *
+  $query = "SELECT *
     FROM {simplenews_mail_spool} s
-    WHERE s.status = %d
-    ORDER BY s.timestamp ASC",
-    $status, 0, $limit);
-  while ($data = db_fetch_array($result)) {
-    $message = array();
-    foreach ($data as $key => $value) {
-      $message[$key] = $value;
+    WHERE " . implode(' OR ', $clauses) . "
+    ORDER BY s.timestamp ASC";
+
+  /* BEGIN CRITICAL SECTION */
+  // The semaphore ensures that multiple processes get different message ID's. So there could not occur any duplicate messages.
+
+  if (lock_acquire('simplenews_acquire_mail')) {
+    // Get message id's
+    // Allocate messages
+    if (is_numeric($limit)) {
+      $result = db_query_range($query, $params, 0, $limit);
+    }
+    else {
+      $result = db_query($query, $params);
+    }
+    while ($message = db_fetch_array($result)) {
+      $messages[$message['msid']] = $message;
     }
-    $messages[$data['msid']] = $message;
+    if (count($messages) > 0) {
+    // Set the state and the timestamp of the messages
+      simplenews_update_spool(array_keys($messages), array('status' => SIMPLENEWS_SPOOL_IN_PROGRESS));
+    }
+
+    lock_release('simplenews_acquire_mail');
   }
+
+  /* END CRITICAL SECTION */
+
   return $messages;
 }
 
@@ -409,7 +469,7 @@ function simplenews_get_spool($status, $nid = NULL, $vid = NULL, $limit = 999999
  *   Mail spool id of record to be updated
  * @param array $result
  *   Array containing email sent result
- *    'status' => (0 = hold, 1 = pending, 2 = send)
+ *    'status' => (0 = hold, 1 = pending, 2 = send, 3 = in progress)
  *    'error' => error id (optional; defaults to '')
  */
 function simplenews_update_spool($msids, $result) {
@@ -433,18 +493,30 @@ function simplenews_update_spool($msids, $result) {
  * @param integer $vid newsletter revision id
  * @param string $status email sent status
  *
- * @return array Mail message array
+ * @return integer count of mail spool elements wich owns the attributes passed in as params
  */
-function simplenews_count_spool($nid, $vid, $status = SIMPLENEWS_SPOOL_PENDING) {
-  return db_result(db_query("
-    SELECT COUNT(*)
-    FROM {simplenews_mail_spool}
-    WHERE nid = %d
-      AND vid = %d
-      AND status = %d",
-    $nid,
-    $vid,
-    $status));
+function simplenews_count_spool($nid, $vid, $status = array(SIMPLENEWS_SPOOL_PENDING, SIMPLENEWS_SPOOL_IN_PROGRESS)) {
+  $clauses = array();
+  $params = array();
+
+  if (!is_array($status)) {
+    $status = array($status);
+  }
+
+  foreach ($status as $s) {
+    $clauses[] = 's.status = %d';
+    $params[] = $s;
+  }
+  $params[] = $nid;
+  $params[] = $vid;
+
+  $query = "SELECT COUNT(nid)
+    FROM {simplenews_mail_spool} s
+    WHERE (" . implode(' OR ', $clauses) . ")
+    AND nid = %d
+    AND vid = %d";
+
+  return db_result(db_query($query, $params));
 }
 
 /**
diff --git a/simplenews.module b/simplenews.module
index ad3d4e3..0493e27 100644
--- a/simplenews.module
+++ b/simplenews.module
@@ -50,6 +50,7 @@ define('SIMPLENEWS_STATUS_SEND_READY', 2);
 define('SIMPLENEWS_SPOOL_HOLD', 0);
 define('SIMPLENEWS_SPOOL_PENDING', 1);
 define('SIMPLENEWS_SPOOL_DONE', 2);
+define('SIMPLENEWS_SPOOL_IN_PROGRESS', 3);
 
 /**
  * AFTER EACH 100 NEWSLETTERS