问题
When I send an email to somebody, (I think) my email is sent to my home server, then the email is sent to target server, then it is finally sent to the people I want to send to.
Sometimes, this needs multiple hops, which is the point that confuses me. Why does this need multiple hops? Why can't the email be sent to the target server directly? XMPP doesn't need multiple hops, for example.
回答1:
Fundamentally that has to do with the original purpose of the Internet, or rather it's predecessor ARPANET, was designed to survive the loss of portions of the network (popular to popular misconception, this design goal existed not in order to survive nuclear attack, but because network components were rather unreliable at the time).
The packet switching network is designed to find one or more routes from where the packet starts (your email client) to where it needs to be (the email client of the person you are sending to).
In order to achieve that flexibility, the system is designed not just to route directly from point A to point B, but to find an available route (from a possibly very large set of possible routes) that is currently functioning.
If your email client had to know how to directly send to every other device in the world, without intervening subnets managing traffic, your computer would need a direct connection to every other computer in the world. That would be prohibitively expensive. Instead, routers inbetween accept packets from you and other people in your area and figure out how to route each packet closer to its destination. That's similar to the method that the airlines use to have regional flights feed hubs.
回答2:
A typical email transaction uses an outbound mail server, which looks up your recipient's incoming server, delivers it there, then that server hands it off to your recipient's mailbox server. That's 3 hops:
When you send a message, you connect from your email client (more properly: mail user agent, MUA) to your outbound SMTP server. This is the first hop, and your SMTP server creates a Received header denoting it for auditing later on. This will include the IP it sees you connecting on, a message ID internal to that specific server, and sometimes some other information.
Your outbound SMTP server then looks up the MX (mail exchange) record for each recipient's domain in DNS and connects to the highest priority relay it can find (the numerically lowest MX priority) for them. This is the second hop. As with any hop, a Received header is added.
The MX relay typically does anti-spam filtering, potentially rejecting the message during the SMTP connection. This is what generates a bounce message, which your outbound SMTP server sends you directly (if it's sent across the internet, it risks being backscatter, a form of spam, because the bounce cannot be proven to be legitimate). Often, anti-spam filtering is done later and the message is dropped silently or else delivered with markup.
The MX relay then hands the message off internally to your mailbox (IMAP/Exchange) server (hop #3). Sometimes, local mailbox server filters are used to handle patterns (or markup from the MX server) and facilitate various actions such as filing suspected spam into your Junk folder.
If you use a mail forwarding service, the MX relay instead passes your message to the forwarding target (another MX relay). This can theoretically go on for quite a while or even loop infinitely, but it should eventually end in a mailbox server.
See also the formal SMTP specification, RFC 5321.
If you use webmail, the web mail server is your email client. Some web mail servers add headers resembling Received headers to track the connection so spam filters can extend their reputation checks to the user rather than to the webmail system, which could be characterized as yet another hop.
In enterprise-grade environments, it is common for multiple layers of servers to be used for various purposes. These extra hops happen between the MX relay and the mailbox server and can be related to extra anti-spam or anti-virus handling, advanced mail aliasing, email archiving, internal mail routing between campuses, or any number of other things. These can for example ease transitions from one mail system to another (like migrating between versions of Exchange, which may require lots of testing and a slow migration so a problem doesn't affect the whole company).
回答3:
When a packet is being routed through the internet it will make multiple hops to get from source to destination.
来源：https://stackoverflow.com/questions/11182990/why-does-sending-an-email-need-multiple-mail-server-hops