Thank you, Steve. That is really interesting. That solves that one for me. Now, can you tell me why sometimes when someone sends an email, it may be time stamped a few minutes to a few hours before it comes in. For instance, their email says 9:02AM and it comes in at 3:23PM (they are in the same timezone).
Again, without going too deep into the technical weeds, the types of oddities you're describing are generally also due to the nature of networks and how the internet works.
When you send an email, your ISP's server has to translate the human readable address into something computers can deal with. There are specific protocols for how this happens and several layers of server hierarchy to go through to find the correct IP address for the intended inbox. Then the message is sent from server to server along one of possibly millions of paths to get there. At each new server, the message is copied and the copy sent to the next server in the chain. There will be dozens, if not hundreds of intermediary servers scattered all over the world, involved in the transmission of one simple email. At each step, there is a remote possibility of that server not functioning correctly. Those malfunctions can be simple transmission errors, that resolve themselves in milliseconds, or it could be something more serious that allows the server to accept new messages and queue them up, but prevents it from sending the messages on. It's this latter situation that results in a message being sent at 9:02am but not arriving until 3:23pm. It took 6 hours for the server to be reset or physically repaired and your message was stuck there until service was restored. And depending on how much traffic that server processes, it could take a long time to work off the backlog once service is restored.
You'll sometimes see the opposite as well - messages "arriving" before they were "sent". This happens when a message is not copied correctly and a server attempts to reconstruct it. When that happens the creation timestamp may be reset by a server several timezones away. So the 9:02am becomes 4:52pm and the message arrives before it was sent.
To complicate things even more, email messages are typically broken up into small, fixed length data packets - meaning your single message may now consist of many pieces, each of which is following a different path with an unknown number of servers involved. The final message is not delivered until all the pieces can be put back together again.
Sorry this is so long, I hope it helps some.
Steve