An email bounce occurs when we attempt to send an email, but the destination mailserver does not accept it. Depending on the type of bounce message we receive, we may log a bounce against the contact. If a contact receives three consecutive bounces, from three separate emails, we will then automatically mark them as a bad address.

In some cases destination mailservers respond with very specific bounce messages, such as “user unknown”, which are considered to be severe enough to immediately trigger a bad address flag against the contact.

We do this to help protect your sending reputation. For example, email providers may identify your sending to known bad addresses as spam-like behaviour; to them it closely resembles how spammers operate. For example, spammers might write a script to guess email addresses To an email provider, a sender hitting lots of these non-existent email addresses looks very suspicious.