Why Does My Email Bounce
If your outgoing email is un-deliverable for any reason, it will bounced back to your [reply] email address. That way you will be aware that it was not delivered to the intended recipient.
Some of the reasons for an email to bounce back to you is
o User unknown
o Mailbox full
o Mailbox unavailable
o Mailbox blocked spam filter
o user complaints
o Invalid recipient
o email address had syntax error
o mailfolder is full
Here are some common replies from recipient SMPT servers:
Subject: Delivery Status Notification
Final-recipient: rfc822; name@domain.com
Action: failed
Status: 5.1.1
Diagnostic-Code: smtp; 550 Mailbox unavailable or access denied
Subject: Delivery Status Notification (Failure)
Final-Recipient: name@domain.com
Action: failed
Status: 5.5.0
Diagnostic-Code: smtp;550 Requested action not taken: mailbox unavailable
Subject: Delivery Failure
Failed Recipient: name@domain.com
Reason: Remote host said: IP_addr temporarily deferred due to user complaints - IP_addr; see http://postmaster.domain.com/421-ts01.html
Subject: Mail Delivery Failure
Diagnostic-Code: smtp; 550 Invalid recipient:
Note that this next bounced email the domain part of the email address is mis-spelled, causing the host domain name to not exist, thus the [host not found] message:
Subject: Returned mail: see transcript for details
----- The following addresses had permanent fatal errors -----
name@domainc.om
reason: 550 Host unknown
----- Transcript of session follows -----
550 5.1.2 name@domainc.om... Host unknown (Name server: wtwerfc.om: host not found)
Reporting-MTA: dns; mail.electrasoft.com
Received-From-MTA: DNS; IP_addr.tx.comcast.net
Arrival-Date: Fri, 18 Dec 2009 23:45:35 -0600
Subject: Returned mail: see transcript for details
550 User suspended: name@domain.com
550 5.1.1 ... User unknown
Subject: Returned mail: see transcript for details
Message could not be delivered for 1 day
Message will be deleted from queue
Subject: Undelivered Mail Returned to Sender
Subject: failure notice
name@domain.com
child status 100...The e-mail message could not be delivered because the user's mailfolder is full.
Subject: failure delivery
name@domain.com
Can't open mailbox for name@domain.com. Temporary error
I'm not going to try again; this message has been in the queue too long.
Subject: Undelivered mail: User unknown
550 : User unknown in virtual mailbox table
Reporting-MTA: dns; mx1.domain.com
A Closer Look at the Reason Emails Bounce
When examining a bounce, first, look at a couple of bounce messages. Buried in the all the tech-ee stuff you will see some important clues:
---- start ---->8
----- The following addresses had permanent fatal errors -----
name@domain.com
reason: 553 sorry, relaying denied from your location [IP_Addr] (#5.7.1)
----- Transcript of session follows -----
... while talking to smtp.example.net.:
)))) DATA
((( 553 sorry, relaying denied from your location [IP_Addr] (#5.7.1)
550 5.1.1 name@domain.com... User unknown
((( 503 RCPT first (#5.5.1)
---- end ---->8
This bounce from another SMTP email server attempts to be friendly:
---- start ---->8
Hi. This is the qmail-send program at domain.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.
name@domain.com:
[IP_Addr]. does not like recipient.
Remote host said: 550 MAILBOX NOT FOUND
Giving up on [IP_Addr].
---- end ---->8
These messages found in the body of the bounced email [MAILBOX NOT FOUND] and [User unknown] may or may not mean the same thing.
Common SMTP Email Server Error Messages:
Mailbox Not Found, invalid mailbox, User unknown, not our customer are all caused by the same problem. In the [name@domain.com] bounce examples above, the mail server [domain.com] doesn't have an account for anyone with the email name [someone]. Here are some common reasons:
o 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.
o The email address an old one that's is no longer in use. Perhaps the person you're attempting to email has changed their email address and you are using an old one which is no longer valid. Make sure you're email address list is up to date.
o Mailbox unavailable: Usually, this is the same as [mailbox not found]. Sometimes it 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.
o 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.com or Yahoo.com, which have limits on how much mail you can accumulate. This can also be a sign of an abandoned account. This person has 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.
o Host unknown, Domain Lookup Failed: This means that the SMTP email server you're attempting to send to, the [domain.com] part, in the examples above, doesn't exist. A common reason is a typo on your part. Make sure you typed it in correctly. Another reason is an ISP (Internet Service Provider) that change it's 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.
o Unable to Relay: This error message is becoming more common as ISPs try to crack down on spam. Email 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 SMTP email server at your recipients ISP.
In general, a mail server must [know] either the sender of an email, or the recipient, in order to safely transmit mail. SMTP email servers that do not enforce this requirement are called [open relays] and can be exploited by spammers to send out 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 [unable to relay] messages. Depending entirely on the servers and ISPs involved: The [From] email address might not match an account on the outgoing SMTP 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 (use EHLO, AUTH LOGIN, username and password) before sending email and you haven't. If this is the problem, read this. An SMTP email 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.
o Temporary Errors: 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.
o Blacklist Filters: 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 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 is 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.
o Content Filters: 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, as I discussed in Why is my mail to this person not getting through?, but some servers will actually send a bounce. Phrases in the bounce message like [Message looks like spam], [keywords rejected by the antispam 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 SMTP 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.
What does wait a while and try again mean?
One of the most common solutions for just about any bouncing email problem, after checking that you're sending to the right address, is to [wait a while and try again]. The email system, while somewhat random, is also somewhat self-healing. If there's an email server with a problem, chances are it'll get fixed or eventually bypassed, especially if it belongs to a larger ISP. For temporary problems, as noted above, SMTP email servers will typically keep trying for up to 4 days before giving up.
A rule of thumb for re-trying emails, is [one hour, one day, one week]. In other words, try again in an hour. There are classes of problems that will resolve themselves that quickly. If that still fails, then try again the next day. If that still fails (and your message can wait that long), try again in a week. If that still fails, you will need to find another way to get the message to your recipient.
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. This scenario is sadly common, and I've written about it in a separate article: Someone's sending from my email address! How do I stop them?
Everything is Bouncing
Finally, if every email you send bounces, then you have a different problem. It could be that your email client is misconfigured. Double check your [SMTP Setup] settings, and double check with your ISP to ensure that you have them set correctly.
Everything is being Rejected
You may accused of breaking some rull set by your ISP or SMTP email server and you have been shut down for spamming. Here is how to figure out why your email is being rejected and how to fix it.
Send email using your SMTP email server without bounce, reject, timeout, authentication errors.
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