Fix an issue when support agent replies to the email notifications an…

…d conversation moved to another mailbox - closes #3455

app/Console/Commands/FetchEmails.php

@@ -561,10 +561,18 @@ public function processMessage($message, $message_id, $mailbox, $mailboxes, $ext
             }
 
             // Make sure that prev_thread belongs to the current mailbox.
-            // It may happen when forwarding conversation for example.
-            if ($prev_thread) {
+            // Problems may arise when forwarding conversation for example.
+            //
+            // For replies to email notifications it's allowed to have prev_thread in
+            // another mailbox as conversation can be moved.
+            // https://github.com/freescout-helpdesk/freescout/issues/3455
+            if ($prev_thread && $message_from_customer) {
                 if ($prev_thread->conversation->mailbox_id != $mailbox->id) {
                     // https://github.com/freescout-helpdesk/freescout/issues/2807
+                    // Behaviour of email sent to multiple mailboxes:
+                    // If a user from either mailbox replies, then a new conversation is created
+                    // in the other mailbox with another new conversation ID.
+                    // 
                     // Try to get thread by generated message ID.
                     if ($in_reply_to) {
                         $prev_thread = Thread::where('message_id', \MailHelper::generateMessageId($in_reply_to, $mailbox->id.$in_reply_to))->first();
@@ -748,6 +756,15 @@ public function processMessage($message, $message_id, $mailbox, $mailboxes, $ext
                     return;
                 }
 
+                // Save user thread only if there prev_thread is set.
+                // https://github.com/freescout-helpdesk/freescout/issues/3455
+                if (!$prev_thread) {
+                    $this->logError("Support agent's reply to the email notification could not be processed as previous thread could not be determined.");
+                    $this->setSeen($message, $mailbox);
+
+                    return;
+                }
+
                 if (\Eventy::filter('fetch_emails.should_save_thread', true, $data) !== false) {
                     $new_thread = $this->saveUserThread($data['mailbox'], $data['message_id'], $data['prev_thread'], $data['user'], $data['from'], $data['to'], $data['cc'], $data['bcc'], $data['body'], $data['attachments'], $data['message']->getHeader(), $data['date']);
                 } else {