Finally, we can send an email. Some can do all kinds of fancy tricks, but we just want to send a message. So, we fire up our email program and start filling in the blanks. Attachments or other fields we can save until later. First, your message is saved on your computer's disk.
Emails take up space.
Notice that your message is both sent and yet remains on your computer. Send enough emails and they will eventually clutter up your disk, but at 1-2k it will take a while. The nice thing is that you have a copy should something happen to the one that was sent. Once the server has the file, it reads it to determine the destination server. Assuming that it can locate the server, it will fire a copy of the message to it.