before stored example

Lua-Examples.md

@@ -35,6 +35,51 @@ function inbucket.after.message_deleted(msg)
 end
 ```
 
+## Event trigger: before message stored
+
+This event fires after Inbucket has accepted a message, but before it has been stored.
+
+Changes the destination mailbox to `test` for two specific recipients.
+
+```lua
+local logger = require("logger")
+
+-- Original mailbox name on left, new on right.
+local mailbox_mapping = {
+  ["swaks"] = "test",
+  ["james-test"] = "test",
+}
+
+function inbucket.before.message_stored(msg)
+  local made_changes = false
+  local new_mailboxes = {}
+
+  -- Loop over original recipient mailboxes for this message, building up list
+  -- of new_mailboxes.
+  for index, orig_box in ipairs(msg.mailboxes) do
+    local new_box = mailbox_mapping[orig_box]
+    if new_box then
+      logger.info(string.format("Mapping mailbox %q to %q", orig_box, new_box), {})
+      new_mailboxes[index] = new_box
+      made_changes = true
+    else
+      -- No match, continue using the original value for this mailbox.
+      new_mailboxes[index] = orig_box
+    end
+  end
+
+  if made_changes then
+    -- Recipient mailbox list was changed, return updated msg.
+    logger.info(string.format("New mailboxes: %s", table.concat(new_mailboxes, ", ")), {})
+    msg.mailboxes = new_mailboxes
+    return msg
+  end
+
+  -- No changes, return nil to signal inbucket to use original msg.
+  return nil
+end
+```
+
 ## Event trigger: after message stored
 
 Prints metadata of stored messages to STDOUT: