@ sign bouncing on a trampolineThis is a continuation (and conclusion) of last week’s post.

More clues to look for in the text of the (rather cryptic) bounce message

Unable to relay or relaying prohibited

There could be several reasons for this message:

  • Your email connection might require authentication by logging onto your incoming server before attempting to send anything out (a setting in the email configuration if you use an email program such as Windows Live Mail)
  • The smtp mail server (ie the outgoing or sending server) that you are trying to use will not allow you to use it as your IP address does not identify you as a legitimate user of that server (for instance, if you are using a laptop with a wifi connection other than your own router at home)
  • The return address of the email message is at a domain other than that of the smtp server


There are dozens of lists on the internet containing the names of servers that are known or suspected to send spam. Your own ISP’s email servers could have got onto one or more such lists. Although completely innocent, your email may be failing to reach its destination because the recipient’s email server found your ISP’s email server on a blacklist they check against.

If this happens to you, you need to contact your ISP, explain that you think they’re on one or more blacklists and ask them to act to get themselves removed. A client of mine fell foul of this problem a few months ago. The reaction of the ISP was somewhat underwhelming in that they said they had tried to get removed but it was outside their control. Not very useful. In the end, we took advantage of a facility provided by Google whereby you can retain your own email address but have your messages routed through Google’s servers. Free of charge and it’s been working ever since (but a bit messy to set up).


This is not as serious as blacklisting. What it means is that the recipient’s email system will only receive email from specific (ie “whitelisted”) email addresses. You would need to contact the recipient by a means other than email in order to get your email address added to their whitelist.

Spam Filters

Look in the bounce message for words such as “blocked” or “suspected spam”

Your email may have been rejected by the recipient’s server as it looked like spam. There are certain things you must avoid when composing emails – eg don’t write in capital letters, don’t use multiple exclamation marks. I was just about to give specific examples of what not to do but then realised that the newsletter version of this blog post would probably get caught up in everyone’s spam filters. The reality, though, is that you are unlikely to know that your message has been rejcted for being suspected as spam. Spam messages aren’t normally bounced – just discarded or held in quarantine.

Non-Delivery Without Bounces

There are times when the recipient claims not to have received your email but you did not receive any bounce message. There are several possible causes of this:

  • Spam Filters – If your email has been blocked by a spam filter then you may not be informed by a bounce message. Your message will seem to have simply disappeared into the ether.
  • Junk Folder – The recipient’s email software (as opposed to the recipient’s email server) may have intercepted your message as spam and moved it directly to the recipient’s Junk folder. So if someone says they haven’t received your email the first thing to do is ask them to check their Junk folder.
  • Email Rules – The recipient may have set up their own rules in their email software to semi-automate the handling of incoming mail. One of these rules may have caught your message and dealt with it in an unexpected way. If you have reason to suspect this, you might ask the recipient to to perform a Windows search on some specific text in your email. If the message is in their system but has been moved somewhere unknown by a rule or filter then this may explain what’s happened.
  • Message or attachment too big – If you suspect this is the problem, try sending a short email with no attachment. If it gets through then it may have been that the incoming server rejected your message because either the message or an attachment was too big. Some email servers won’t accept attachments bigger than 5mb, others 10mb. If I’m sending someone a large attachment I always ask them to confirm receipt.

Conclusion: although bounces are a nuisance, the bounce messages often contain useful information – you just need to glean the information and ignore the machine-speak.

@ sign on a trampoline - bouncing email…..and what you can do about it

What is a Bounce?

If an email message can not be delivered to the inbox of its intended recipient then it is said to “bounce” – ie the sender receives a message advising that delivery failed.

Bounces that aren’t

The first thing to do when you receive a bounce message is to identify the message that wasn’t delivered. There will be a reference to it in the bounce message. If there is no reference to any message originated by you then be careful as this may be spam or a virus and not a bounce at all. In particular, don’t open any attachment if you’re not sure that this message is actually a bounce relating to a message you sent.

Another possibility of a bounce message that did not originate with a message sent by you is known as “backscatter“. Spammers are able to make their messages look as if they came from completely innocent and legitimate email addresses (eg yours). If the spam they send out is bounced back then you will receive that bounce even though you had nothing to do with the original message. It’s an unsettling experience, but all you can do is delete the bounce message.

Real Bounces

A real bounce will refer to a message you sent. If it is a “hard bounce” (the message was rejected by the email server to which it was sent) then you will probably receive the bounce within a minute or so of sending the doomed message. If it is a “soft bounce” (accepted by the email server but ultimately undeliverable to the recipient) then it may be days before you receive the bounce as the server may have made several attempts to deliver it.

To determine what you can do about a bounced message, you need to look for intelligible phrases in the bounce message:

Some common phrases to look for amongst the gobbledegook are:

user not found
not our customer
mailbox not found

All of the above – and others like them – are suggesting that the recipient’s server accepted the message but then couldn’t deliver it to the user because there is no valid user with that username. The username (more properly known as the “local mailbox part”) is the part of the email address before the “@”, so in “fredsmith@example.com” the user (local mailbox part) is “fredsmith”. The cause of this error is very likely to be just a spelling mistake or typo (wrong key hit) on your part. Alternatively, the email account may have been closed so that email address won’t accept any more messages.

The pedant in me insists that I point out that, in theory, the local mailbox part is case sensitive. In other words “FredSmith” is not the same as “fredsmith”. In practice, I have never come across an email failing to get through for this reason. Bizarrely, the “domain name” part of the address (the part after the “@”) is not case sensitive, so “Example.com” is the same as “example.com”.

If the bounce message includes a phrase such as

quota exceeded or
mailbox full

then the user has filled up the disc space that they are allowed to use for email and must move or delete some of it before they can accept more email. If you need to get your email through then it’s often quickest to phone the recipient so that they can do something about it. This is an example of a soft bounce. The server may attempt to deliver the message for two or three days before telling you that it failed.

Another common explanation for a bounce is given as

Host unknown

This either means that the domain name (the part of the address after the @) is incorrect or the server of that name is unavailable. For example, the “example.com” part of the email address “fredsmith@example.com” may be incorrect. Check that you’ve got the email address correct and try again. It could just be that the email server is temporarily busy or unavailable. In that case, sending the message again may result in a normal delivery. If I’ve been having a problem like this, but then the message doesn’t bounce on a re-try, I will sometimes send another message asking the recipient to confirm delivery of the first one. If, however, your second attempt results in a second bounce and you are sure that the address is correct then try a bit later (say, an hour or so). If you haven’t managed to get it delivered in a day then it’s probably best to contact the recipient.

There are other reasons for bounces and sometimes a message doesn’t seem to reach the recipient even though you don’t receive a bounce. I’ll be returning to this topic next week.

© 2011-2019 David Leonard
Computer Support in London
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