Bounces are emails that are returned to the sender, because they cannot be delivered for a given reason. There are two kinds of bounces: hard bounces and soft bounces. A hard bounced email is permanently bounced back to the sender because the address is invalid or some form of unexpected error occurred during delivery. A soft bounced email is recognized by the recipient's mail server but is returned to the sender because the recipient's mailbox is full, the mail server is temporarily unavailable, or the recipient no longer has an e-mail account at that address.
Bounces can be a sign of a stale list. You can check this link for information on how to clean your list. You should also check for the following:
- Look for email addresses that are older than 6 months and remove them from your list.
- Remove any recipient that you cannot show proof of permission for sending.
When you send an email to someone and it bounces back, the server that bounced it back to you will usually include a little message in the email called an "SMTP Reply" (which is meant to explain why the server bounced it back to you). That information is located in the header of the bounced message.
When MailChimp gets a bounced message, we try to read its SMTP reply, then categorize the email as a "soft" or "hard" bounce, so you don't have to do a thing. MailChimp will automatically unsubscribe hard bounce addresses.
But the system isn't perfect. Some email servers don't send bounces with properly formatted SMTP replies that use standard codes.