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 -----
email@example.com, firstname.lastname@example.org, email@example.com
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
Not our customer
|Wrong Email address entered
||The email address you are trying to send to is no longer needed
Mailbox not found
|This may be a problem with the recipient's email account
||Check to make sure you are sending to the correct email address. Wait a while and then try again, if it still bounces you may need to try another method of contact.
|Your contact has too many emails and needs to make some space before they can recieve yours. This is commonly found on accounts such as Gmail or Hotmail, where account size is limited.
Can also be a sign of an abandoned account, where your contact may have stopped checking their account and caused their email account to fill up.
|Wait for your contact to empty their email account, or try and contact them through another method.
Domain look up failed
|This means that the example.com part of the email address does not exist; either through a misspelling or the ISP changing its name. For example, attbi.com changed its name to comcast.com, causing any emails sent to attbi.com addresses to be bounced back.
|Check that the email address you are sending to is up to date, and accurate.
|Unable to relay
|This is a blanket term for a host of different errors that can be possible, such as:
|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.
|No adequate servers
Connection timed out
Resources temporarily unavailable
Out of memory
|These typically related to 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
|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
||Your email looks too much like a spam message and has been blocked by the content filters.
||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.