This answer explains what all that geeky text means when you get a returned email.
Common Bounce backs
There are many reasons mail could bounce.
Bounce messages can vary in format, and in exact wording, depending on the mail server that's sending the message back to you. Different types of mail servers use different terminology. Some are quite geeky and difficult to understand. Others seem to take five paragraphs to tell you that you probably just mistyped the email address you were sending to.
Understanding a Bounce back message
First, let's look at a couple of bounce messages. Buried in the all the geekery, is highlighted a couple of important things:
(reason: 553 sorry, relaying denied from your location [10.10.10.10] (#5.7.1))
----- Transcript of session follows -----
... while talking to smtp.example.net.:
<<< 553 sorry, relaying denied from your location [10.10.10.10] (#5.7.1)
550 5.1.1 ... User unknown
<<< 503 RCPT first (#5.5.1)
Here's a bounce from another mail server which attempts to be friendlier:
Hi. This is the qmail-send program at example.com.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.
10.10.10.10. does not like recipient.
Remote host said: 550 MAILBOX NOT FOUND
Giving up on 10.10.10.10.
----- The following addresses had permanent fatal errors -----
The messages "MAILBOX NOT FOUND" or "User unknown" are key ... and might actually be any of several different messages depending on the reason for the failure.
Common Error Messages
Mailbox Not Found, invalid mailbox, User unknown, not our customer: these are all saying pretty much the same thing. In the "email@example.com" bounce examples above, the mail server "example.com" doesn't have an account for anyone with the email name "someone". A couple of common reasons:
You typed the email address wrong. The single most common reason this error happens is simply that you made a typographical error in the email name. Check the entire email address for an error.
It's an old email address that's no longer in use. Perhaps the person you're attempting to email has changed their email address, and you're using an old one which is no longer valid. Make sure what you're using is up-to-date.
9 times out of 10, this is the same as "mailbox not found". That other 10% of the time it could mean that there's a problem with the recipients email account. What kind of problem is hard to say. Check to make sure that you have the email address correct, wait a while and try again, and if it still bounces, try contacting the recipient some other way.
Mailbox full, or Quote Exceeded:
Sometimes this will show up as a part of a "Mailbox unavailable" message. It's fairly clear, though: your recipient has too much email and their server isn't accepting any more. This is most common with web-based email services like Hotmail or Yahoo, which have limits on how much mail you can accumulate. This can also be a sign of an abandoned account - someone's stopped looking at and cleaning out the email. In any case, you'll need to try and contact your recipient through some other email account, or some other way.
Host unknown, Domain Lookup Failed:
This means that the mail server you're attempting to use, the "example.com" part, in the examples above, doesn't exist. A common reason is again, a typo on your part. Make sure you typed it in correctly. Another reason are ISPs that change their name. The largest example of this in recent memory has been "attbi.com" changing their name to "comcast.com". Anyone trying to send to an old "attbi.com" email address might get this message in return.
Unable to Relay:
This is a terribly obscure error message, but also becoming more and more common as ISPs try to crack down on spam. Mail is sent by relaying email from one server to the next. There could be many servers involved, but typically it's the mail server at your ISP relaying your email to the mail server at your recipients ISP.
In general, a mail server must "know" either the sender of an email, or its recipient, in order to safely transmit mail. Mail servers that do not enforce this requirement are called "open relays" and can be exploited by spammers to send out tons of spam.
Things get complicated because not all ISPs agree on what it means to "know" the sender of an email. All of these might result in an "unable to relay" message, depending entirely on the servers and ISPs involved:
The "From" address might not match an account on the email server.
The ISP might require that email comes via a connection (dialup or DSL) actually provided by the ISP - sending using someone else's connection might not be allowed.
The ISP might require you to authenticate before sending email and you haven't.
A mail server somewhere could be misconfigured.
There's no blanket answer if "unable to relay" happens only occasionally. Double check the email address you're sending to, for starters.
Errors like "no adequate servers", "Connection Timed Out", "Resources temporarily unavailable.", "Out of memory" all typically indicate a problem with a mail server that you probably don't have any control over. They are, in general, temporary, and should resolve themselves over time. Look carefully at the bounce message; the email server involved may continue to automatically try to deliver your email without any action required on your part.
If you see messages that indicate your email was "blocked", or "listed in", and references to sites that have things like "spamcop", "dynablock", "blackhole", "spamhaus" and similar in their names, then your email was probably intentionally blocked because the receiving system thinks your ISP's mail server is a source of spam.
Various blacklisting services try to identify servers which are sources of spam. They then make that list available to ISPs, who in turn can block email coming from these sources. The problem is that criteria for addition and removal from these blacklists are vague, at best, and getting a server removed from blacklists can be very difficult. If this happens to mail you send, get in touch with your ISP and explain that their server may be on a blacklist somewhere, and then try to use a different email address, or a different email account of your own, to contact your intended recipient. You might also tell your recipient that their ISP is improperly blocking legitimate email.
Much like blacklists, content filters are an approach many ISPs now implement to stem the tide of spam for their clients. Most will simply discard email that looks like spam, but some servers will actually send a bounce. Phrases in the bounce message like "Message looks like spam", "keywords rejected by the anti-spam content filter", "scored too high on spam scale" and similar means that your email, for whatever reason, tripped the spam filters on the receiving end. Your email looks too much like spam.
What does it mean to "look like spam"? Here, again, things get vague. That definition will vary greatly based on how your recipient's email server has been configured. Obvious possibilities are the use of pornographic words or phrases, HTML formatted email, currently popular drugs being hawked by spammers, or even having something that looks too much like a sales letter or a scam. The best approach is to scan the bounce for any clues (sometimes there's more information), and then validate your recipient can get any email by sending a simpler message. Assuming that all works, then re-work your message as best you can to not look like spam.
When a Bounce Isn't Really a Bounce
Be careful! There's a class of viruses these days that propagate by "looking like" bounce messages. They instruct you to open an attachment for more information. Don't. Especially if you don't recall sending the message in the first place. Don't open any attachment, especially one accompanying what looks like an email bounce unless you are absolutely positively certain that it's legitimate.
You may also be getting bounce messages for email you didn't send. There's another class of virus that "spoofs" or fakes the "From" address on email messages, and as a result you could be getting bounce messages that have nothing to do with you.
Finally, if every email you send bounces, then you probably have a different problem. Chances are your email client is mis-configured. Double check your outgoing or "SMTP" server settings, and double check with your ISP to ensure that you have them set correctly.