Sometimes it feels like there is only one reason that your email didn't get through to its recipient, and that reason is "it's all bunn's fault". However, occasionally there are Problems On The Internet that are not down to me and the cleft stick with which, of course, I personally hand-deliver all emails that are in any way related to any organisation that I work with, or have heard of. Here are some of them.
1) Sender mistyped the email address.
Nobody ever, ever admits to this (unless you take them through 999 questions about their email client and setup, and they start to worry that they will be late for lunch). And yet, we all do it. We typed .org, not .org.uk. We mispelled the person's first name, or stuck an 's' on the end of the organisation name. We missed a hyphen, or selected the wrong Reverend Jobias Higginbottom from the dropdown list.
Embrace your inner idiot, and just send the damn thing again to the right person this time.
2) Sender attached Ginormofile
Many, many email systems put limits on the size of attached files they can accept, and if you send an email with a bigger file on it, they will quite often eat it, burp silently, and not tell anyone.
These limits depend on the preferences of whoever set up the receiving (and occasionally also sending) email system, so they vary. No, I can't give you an idea of what 'an average system' will accept, sorry. They are all different. Don't send huge files by email and expect them all to arrive. Definition of 'huge' is : it didn't make it.
3) Recipient didn't actually want to do/have time to do what you asked them to do, so deleted the email and blamed technology.
Of *course* this happens? You really thought it didn't?
4) Sender lied when they said they had already sent the email
Oh go on. You've never done this? Not even once?
5) Daily or size limit on recipient's mailbox. I've never actually come across a live example of a mailbox with a daily limit on it, but I understand they are out there. Size limits are everywhere. If someone just sent your recipient a not-quite-Ginormofile, then their poor little mailbox may be bulging at the seams already, and not have space for even the most slender and well-crafted of text-only messages. Email again tomorrow. Or phone them.
Your email had a virus or other nasty (eg, maybe a link to a scam you fell for yourself?) attached to it, and thankfully something has spotted that and got rid of it before it could infect Aunty Maude's computer or cause her to send thousands to Nigeria, the poor dear old soul. Check your virus protection.
7) Not Connected to Internet
Just have a quick look and check you can browse the web and email other people. If you can't, that *just* might be why your email didn't go. (Yes, I admit, I have been caught by that one myself...)
8) Mailserver problems
Unusual, but happens occasionally. Most emails travel through at least two servers - an SMTP server, which is the one that does the Sending, and a POP or IMAP server, which is where the email is sent to and stored until the recipient comes by to collect it, rather like a random item in the window of Bagpuss's shop. (Or like a Royal Mail PO box, I spose, but I like the Bagpuss parallel better.)
Sometimes there are other servers in between. Occasionally, one of these machines falls over. Usually when that happens, some backup system kicks in, and the email gets where it is going after a delay, but occasionally it doesn't. Rare.
9) Wandering Stranger Eaten by Internet Goblins
These are the ones that really confuse people because they are so unpredictable. People often come up with odd theories and superstitions to try to 'guarantee' their email will escape, or get very angry when messages don't arrive.
There are a lot of systems running quietly unseen, filtering out spam and viruses and other crud, in order to make email useable for everyone. Sometimes they get it wrong. Or maybe they just get hungry, who knows? Even if a system in theory sends all spam and unwanted emails through to a spam folder for your enjoyment, some messages will probably be detected and munched before they make it there. If those are messages you didn't want to see, you'll never notice. But if you did want them...
Here are some factors that are particularly liable to make the Internet Goblins hungry - though exactly why some emails that seem no use to man nor beast end up in your inbox and some that you actually wanted to receive died unknown and unmourned, somewhere on the internet, I cannot tell you. I'm not sure anyone can, though anyone who wishes to have a go in the comments is applauded.
A) Norty Words
Most people know that spam filtering uses lists of rude words to try to identify and remove spam (or in the jargon: UCE - unsolicited commercial email). But you know, some people really *are* looking for that elusive solution to erectile dysfunction, and those people use email too. So, many spam filters will learn as you use them. If you send your unintentionally-filthy message to both Amy and Zoe, and Zoe receives the message and Amy doesn't, it might be because Zoe rather likes that sort of thing and has had similar messages before. Or it might not, of course. Best not make assumptions.
B) You ain't from round here boy
If you email Zoe all the time and she emails you back, then you tend to find those emails will rarely go astray, because filtering systems often let existing contacts go through. They are more likely to eat new, one-off emails from strangers, because being new and unheardof is spamlike behaviour.
This produces the annoying phenomenon where you and your family and friends have been using email for years to stay in touch, and never lost a message. Then, you get a job where you need to receive emails from Joe Public, and woe! Joe Public's messages sometimes vanish into cyberspace! Most people conclude at this point that their work mailbox is broken, and complain loudly. But what is really going on is that the home mailbox mostly receives replies from known contacts and has had time to learn what sort of messages you like, whereas the work mailbox is new, has more robust filtering, and receives messages from unknown weirdos.
C) Stay your swords! I have a letter of safe passage!
It is possible to set up a domain name so that some machines are specially authorised to send email from it.
Say your email is firstname.lastname@example.org.
You can set up the movingcastle.co.uk domain name so that only the IP address used by your own mailserver Calcifer is allowed to send email from it, by setting up an SPF record.
This means that internet goblins will check all emails from that domain name are being sent via Calcifer. If they aren't, they get eaten. But if they are, then the internet goblins will cry 'Pass Friend and All's Well' and suddenly your legit email is much more likely to get where it's going. There are other authentication systems too, some proprietary, some open source.
The existence of these authentication systems explains why you got that annoying message about mobile phone offers just fine, but not the important email from Amy about going to the pub later. It's fairly easy for large corporations to set up authentication systems and make sure their emails all use them. It's much harder for home users, particularly home users who want the freedom to send email from phones, from cafes, and while on holiday in Bermuda.
Basically : Email. Sometimes it gets there. Sometimes it doesn't.