沒有什麼比有人給你發一封重要的電子郵件更令人沮喪的了，你從來沒有收到過，而且任何一方都不知道它發生了什麼。今天的《超級使用者問答》關注的是可能會阻止電子郵件到達預定目的地的問題。

今天的問答環節是由SuperUser提供的，SuperUser是Stack Exchange的一個分支，是一個由社群驅動的問答網站分組。

圖片由香港郵電聯（Flickr）提供。

問題

超級使用者讀者otisonoza想知道電子郵件是如何丟失的：

This happened to me yesterday. Someone claimed she sent me an e-mail, but I did not receive it. It is not in my Inbox, Spam, Trash, etc. It is nowhere to be found. How is this possible (assuming there is no user error)? Where can things go wrong along the way?

電子郵件在傳送過程中可能遇到哪些問題會阻止它進入某人的收件箱？

答案

超級使用者貢獻者Sammitch和Hennes為我們提供了答案。首先，Sammitch：

Tracing the path from the sender to you:

1. It never actually sent. A lot of people do not even notice that a message is sitting in their outbox, unable to be sent for any number of reas***.

2. The mail client successfully sent it to the SMTP server, but the SMTP server has not been able to forward it on to the next hop.

• The SMTP server might be so busy that it has a backlog of messages to process, there might be a delay of several hours.
• The SMTP server might have tried to send it, but the receiving server ‘could not/would not’ immediately accept it (deferral). The sending server will continue to attempt delivery, and most servers will do this for up to two days or more before bouncing the message back to you as undeliverable.
• The receiving server may have rejected the message outright (blacklisted, spam scan, mailbox full, non-existent user) and the sending server either can not or will not send a bounce message back to you.

3. The message was accepted by the receiving server, but…

• The receiving server is backlogged and the message is sitting in a queue waiting to be processed/delivered.
• The message was flagged as spam and dropped. This is bad practice since the message should have been rejected outright, but many servers do this (I suspect Gmail of doing this from time to time).
• The message was somehow undeliverable and either the server is configured to not send a bounce message, or the bounce message itself is undeliverable.

4. The message was delivered somewhere in your account, but…

• Your e-mail client has not properly synced with the server. Close and reopen it.
• You are not looking hard enough. I know this sounds petty, but the majority of the time this is it and it is incredibly frustrating to resolve because people take insult in being asked to double check something so simple that they “could not possibly be wrong”.

Source: I administrate e-mail servers.

Because the majority of person-to-person personal e-mail messages flow easily through the mail system and are delivered near-instantly, people take that speed for granted and treat e-mail like an instant messenger. Under certain circumstances your perfectly legitimate, 3-word e-mail might take several minutes, hours, or even days to be delivered.

Be patient.

接著是海恩斯的回答：

Things can go wrong in lots of places.

Mail follows a path from server to server. One of those could have crashed after receiving the mail, but before passing it on.

Or it could have been identified as spam. Depending on your source, 95% to 98% of all e-mail is undesired spam. Some of those are recognized and put into a special folder. Some of them are simply dropped without notification. I have had this happen to me with scanned documents (from an MFC ‘printer’ which ‘scanned to a PDF e-mail’) at the time when PDFs were popular with spammers.

We eventually tracked down the problem after sending simple test e-mails containing only raw text, they arrived, but anything with only a PDF failed to arrive. For this, you would need the help of the people managing the receiving mail servers, and they will ask you some questi*** such as the exact time you sent your e-mail (without that they need to go through a lot of logs). With the precise time, they can at least confirm if the e-mail was received or not.

Needless to say, ask the user to look in their spam folder before raising a problem with the relevant postmaster.

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