How can an e-mail get lost?
Tracing the path from the sender to you:
It never actually sent. A lot of people don't even notice that a message is sitting in their outbox, unable to be sent for any number of reasons.
The mail client successfully sent it to the SMTP server, but the SMTP server hasn't 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 couldn't/wouldn't immediately accept it. [deferral] The sending server will continue to attempt delivery, and most servers will do this or 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-existant user] and the sending server either cannot or will not send a bounce message back to you.
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.
The message was delivered somewhere in your account, but...
- Your email client hasn't properly synced with the server. Close and reopen it.
- You're 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 "couldn't possibly be wrong".
Source: I administrate email servers.
Because the majority of person-to-person personal email messages flow easily through the mail system and are delivered near-instantly people take that speed for granted and treat email like an instant messenger. Under certain circumstances your perfectly legitimate, 3-word email might take several minutes, hours, or even days to be delivered.
Be patient.
Things can go wrong in lots of places.
E.g. 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% all email is undesired spam. Some of those are recognized and put into a special folder. Some of them are simply dropped without notification. I've had this happen to me with scanned documents (from a MFC 'printer' which 'scanned to a PDF email') at the time when PDF's were popular with spammers.
We eventually tracked down down the problem after sending simple test email only containing raw text did arrive 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 questions such as the exact time you sent your email (without that they need to go through a lot of logs. With the precise time they can at least confirm if the email was received or not).
Needless to say, ask the user to look in their spam folder before raising a problem with the relevant postmaster.