A bounce occurs when the ContentMX mail server is unable to find a destination or receives feedback from the destination server that the email cannot be delivered. The most common reason for a bounce is that the email box no longer exists.
Bounce rates can vary depending on a number of factors including the age of the list, how frequently you are emailing the list, and the method used to build the list.
If you send to a mailing list that has not been used in a few months or more, the bounce rate will be much higher when you send your first mailing. There is a natural churn on an email list that will build up over time. Once you start sending your mailings on a regular basis, the bounce rate should return to a normal level.
Why is the number of emails delivered so much less than the number of recipients?
Over time, you will also notice that the number of emails delivered will become increasingly less than the number of recipients to which you are sending your message. This occurs because previously bounced email addresses that are “permanent bounces” are automatically considered “undeliverable” by the server and subtracted from a mailing. For example, if you have a list of 1,000 recipients and 20 emails bounced on the first mailing and then 10 emails bounced on the second mailing, on the second mailing the delivered statistic would be somewhere around 970.
NOTE: Do not try to do the math. The mail server reports delivery confirmations and bounces separately. It is possible for a message to produce no bounce or delivery confirmation. It is also possible for an email to “temporarily” bounce or be blocked by a spam filter. In this case, an email might bounce the first time, but be successfully delivered the second time.