Why Does Email Bounce?

@ 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.