When you send an email that cannot be delivered, it’s called a bounce. When this happens email service providers, such as Mailchimp, will provide a reason for the issue and will categorise it as either a hard bounce or a soft bounce.
Hard bounces indicate a permanent reason that will prevent an email from being delivered This is usually caused by an invalid email address or an unexpected error during sending. In most cases, email service providers will automatically remove hard bounced contacts from your audience so that they cannot be included in future email campaigns.
Soft bounces indicate the target email address is correct, but there is a temporary issue preventing the email from being delivered. Soft bounces are recognised by the email server but are returned to the sender for a variety of reasons. This can include issues like the mailbox being full or temporarily unavailable.
If an email address repeatedly soft bounces in future email campaigns, it will eventually be changed to a hard bounce.