What Are Bounce Messages?

When we send email messages, sometimes it doesn't reach our intended recipient for a number of reasons. When that happens, you will receive a separate message stating that your message has not been delivered along with an explanation for that. This email is called a bounce message.

Why does email bounce?

There are several reasons why an email could bounce. One of the reasons could be a simple misspelled or mistyped recipient's email address.

That email that you receive after your initial email failed to reach its recipient/s contain a lot of nerd-friendly terms, but underneath all that nerd-gibberish are the true reasons why your mail was undelivered. Here are some of the most common reasons:

To cite examples of bounce messages:

   ----- The following addresses had permanent fatal errors -----

    (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.:
>>>> DATA
<<< 553 sorry, relaying denied from your location [10.10.10.10] (#5.7.1)
550 5.1.1 <somewhere@example.com>... User unknown
<<< 503 RCPT first (#5.5.1)

Here's a bounce from another mail server which attempts to be more friendly:

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.

Mailbox Not Found, invalid mailbox, User unknown, not our customer

All of these basically mean the same thing: the mail server (in this case, that's the 'example.com') doesn't have an account for a user named (in this case) 'someone'. The reason/s could be:

• A simple typographical error. You could have mistyped or excluded some characters in the email address of your intended recipient. Doublecheck the email address of your intended recipient for errors.
• An old email. Old emails get invalidated. Your intended recipient might have changed his/her email address, and you could be using an old, invalid email address. Keep your address book up-to-date.

Mailbox unavailable

More often than not, this is similar to "mailbox not found". In some cases though, there might be a problem with your recipient's email account. It's hard to say what problems specifically. What you can do is to make sure you got the right email address, wait for a while and try sending your email again. If it still bounces, contact your intended recipient some other way.

Mailbox Full or Quota Exceeded

Both mean the same thing: your recipient has too much email and cannot allow any more mail to get through. This is common in web-based email services. They set limits on how much mail you can amass. A mailbox full of messages may also be a sign of an abandoned account - someone has stopped checking and cleaning out emails.

Host unknown, Domain Lookup Failed

That means the "example.com" part does not exist. One reason could be a typo. Another reason is an ISP that changed its name. For instance "attbi.com" changed its name to "comcast.com." If you try to send an email to an attbi.com account, you may receive this message in return.

Unable to relay

This message is becoming more and more common as ISPs attempt to crack down on spam. Usually, the mail server at your ISP relays your email to the mail server at your recipient's ISP. Generally, a mail server must 'know' the sender or the recipient of an email to safely transmit mail. Servers who do not implement this requirement are called "open relays". Spammers take advantage of open relays to send out tons of spam.

The problem, not all ISPs agree on what "knows" the sender of an email means, thus all the following may result in an "unable to relay" message (depending solely on servers and ISPs involved):

The sender or "From" address might not match an account on the email server.

The ISP might require an email to come via a connection (DSL or dial-up) actually provided by the ISP (sending an email using someone else's connection might not be allowed).

The ISP might require you to authenticate before sending an email and you did not.

Some mail server might be disconfigured.

Temporary errors

Messages such as "no adequate servers", "Connection Timed Out", "Resources temporarily unavailable", "Out of memory" all mean that there might be some mail server problem that you have no control over. Read the message carefully; the server may continue to automatically try to deliver your email without you having to do anything.

Blacklist filters

This happens when the receiving system intentionally blocked you for thinking your ISP's mail server is a spam source. You may receive messages saying that your email was "blocked" or "listed in", followed by references to site that have names like ""spamcop", "dynablock", "blackhole", "spamhaus", or similar looking names. 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.

Content filters

Content filters function like blacklist filters. Many ISPs now enforce content filters to prevent block spam for clients. Content filters usually just discard email that looks like spam. Some will send a bounce email with messages like "Message looks like spam", "keywords rejected by the antispam content filter", "scored too high on spam scale" and the like. Your email, for some reason, signaled to the spam filters on the receiving end.

What does "looking like spam" mean? There's no single definition for this. It varies depending on how your recipient's email server is configured. Possibilities include: use of pornographic words or phrases, HTML formatted email, drugs being peddled online by spammers, or even emails that look like sales letters or scams. Check the bounce email message again for clues. Send a simpler message to validate your recipient. Assuming that works, re-work your message so it won't look like spam.

How long is "a while"?

Sometimes, when these things happen, the most common solutions is to do absolutely nothing. Just wait for a while, check to see if you got the right email address, and try again. Servers with problems get fixed eventually. If not fixed, then bypassed. But exactly just how long does "a while take"? For temporary problems like those discussed above, serves usually try for up to 4 days before they give up.

If your email can wait, you can try sending after an hour. If not, after one day. If not, try after a few days. If after a week you still haven't sent your email, find some other way to contact your recipient.

When is a bounce not really a bounce?

The usual suspects for bogus bounce messages - virus. Some viruses propagate by looking like bounce messages. Some require you to open an attachment. Don't. If you don't remember sending message in the first place.

You may also receive bounce messages for emails you did not send. Another type of virus fake the "From" field on the email messages. As a result, you could receive bounce messages that have nothing to do with you.

What if everything just bounces?

You may have another problem here. If every email you send bounces, then you have a problem with your email client. Your email client is most likely misconfigured. Re-check outgoing or "SMTP" server settings, and check again with your ISP to ensure that you have them set correctly.