Basically mail works in the process of Queue.

• At first, you have typed your message with some attachments and click send button in your Email client.
• It will be moved to the email server (SMTP) and queued that mail.
• Now it will communicate with the respective Recepients DNS .
• Once it is enabled, it will transfer the message to that recepient email server.
• Thus the new mail is downloaded to the email client and the message with attachments is shown to you.