A bounce is when an email is rejected for delivery. The reason is usually apparent in the description of the bounce type. Bounces are broken down into two classifications: soft and hard. Each of these types of bounces are used by remote sites to better manage their email infrastructure. Even though there are two different types, a bounce is a bounce. In other words, the email was rejected.
This type of bounce happens live during the attempt to deliver the email to the remote server. Email delivery is defined by using the SMTP protocol. This protocol is a series of commands that tells a remote mail server what to do, who to deliver the email to and what happens next. In the process of giving these commands to the remote server, the server could return with an error code rejecting the email. For example, a 550 error code describes when a user’s account does not exist or is otherwise permanently not available.
A soft bounce describes an offline bounce. For example, our delivery server will connect to the remote mail server and, again, use the SMTP dialog to deliver a piece of mail. This time, the mail is accepted without any errors presenting. However, simply because a piece of email is accepted for delivery, doesn’t necessarily mean it will ultimately be delivered. As a result, a mail delivery failure could be returned between 1 minute and many days later from the original delivery. In many cases, the soft bounce uses the same error codes as a hard bounce.