Email bounces

What is a bounce

Whenever an email is sent, be it from the web form to the autoresponder or the autorepsonder to the mailing list or just a regular non autoresponder email , the email is either delivered successfully or delivery fails for some reason. This failure to successfully transfer an email from sender to recipient is known as a bounce.

Bounces are an inevitable part of email communication. One advantage of a private autoresponder as apposed to an autoresponder service is that you control how these bounces are handled. This is obviously influenced by the nature of the bounce. Soft bounces are usually temporary in nature and generally easily remedied. Hard bounces are far more serious and require immediate attention.

Why do emails bounce

All emails follow a similar route whether they are handled by autoresponders or not. They need to get from the sender to the recipient passing a number of checks en route. Failure anywhere along the transfer path has the same result -- the email bounces but not all failures are equally 'bad'. There are two distinct stages in this transfer. First the sent email has to arrive at the recipients mail server and be accepted there. Non acceptance by the recipients mail server causes a hard bounce. Next the email needs to continue on from the recipients mail server to the recipient. A failure along this part of the transfer results in a soft bounce

Hard bounce

An email that didn't make it to the recipients mail server is bad news and normally indicates a permanent problem with the recipient details (the email address) or with the sender. With a hard or synchronous bounce the sender gets the bounce message very quickly. Usual causes of hard bounces are;

The domain name or host is unknown (doesn't exist or the spelling is incorrect)
The recipient at that domain is unknown.
The recipient mail server is refusing email from the sender (your email server i.e. a blacklisting issue).

None of the above can right themselves so the problem is permanent until some action is taken. Before you can act you need to know why the messages bounced so it is important to read your bounce messages - never just delete them. There is little uniformity in bounce error codes or descriptions except that the first digit of a hard error code is always the number five. The format might be e.g. 5.1.0 or 550 and the message something like
The reason for the problem: 5.1.2 - Bad destination host 'DNS Hard Error'
The reason for the problem: 5.1.0- Unknown address error 550-'Administrative prohibition'

It is possible to receive a 'false' hard bounce so if you're sure of the validity of the address etc then resend once only the after a gap of at least a day.

Soft bounce

If the email reaches the recipient mail server but not the recipient it normally indicates a temporary hitch. Likely causes include:

The recipient mailbox is full
The receiving mail server disconnects while sending the message
Message size too large

A soft or asynchronous bounce usually occurs when the email server is temporarily unable to deliver the message and a fair bit of time can elapse between the original sending of the message and receipt of the bounce. The first digit of the 3 digit soft error code is always the number four.

Unlike hard bounces these errors are generally temporary in nature so it is possible for a soft bounced message to be successfully delivered on subsequent attempts. However do not attempt to resend before checking the error message. Sometimes human intervention will be required e.g. if the message size is too large. To be on the safe side limit the number of resends to a soft bounced address to 3 in a week and if the problem persists treat it as an invalid address.

Handling bounces with autoresponders

Special email addresses can be set up in the autoresponder to receive these bounce messages but the need for human monitoring and intervention where necessary still exists.