A bounce occurs when an email delivery attempt fails. For example, a "soft" bounce is when a temporary error occurs during a send, that does not remove the recipient from the mailing list, whereas a "hard bounce" happens when an email address or domain is determined to be unreachable, and is removed from the mailing list. There are different types of bounce reasons, and each type will help you identify the best way to manage and determine a course of action for the health of your mailing list.
Types of bounces
Soft bounce (bounce_sb)
When an email reaches the recipient's mail server but returns as undelivered.
Sometimes, the recipient's inbox might be full, which means it may be delivered later.
Please note that three delivery attempts will result in a hard bounce.
Transient bounce (bounce_tr)
When the sender's email server generates a transient bounce, it informs the sender that the message wasn't delivered but that the server is trying again.
The transient bounce will continue for two days after the message has been sent.
When a transient bounce happens, you may receive a notification stating, "Warning: message still undelivered after 4 hours."
However, there is usually no need to worry about transient bounces.
Address Change (bounce_ac)
An address change response indicates that the recipient has changed their address and is notifying senders of the change.
AutoReply (bounce_ar)
An autoreply notification happens when the recipient's email client sends an autoreply to the sender, such as an out-of-office notice.
Unlike other bounces, autoreply notifications indicate that an email recipient is temporarily unavailable.
Autoreply notifications are helpful when sending time-sensitive information.
When email is not time-sensitive, autoreply notifications can be ignored.
Challenge/Response (bounce_cr)
A challenge/response reply is a message sent by special filtering software that only accepts messages from known senders.
This type of filter automatically sends a reply with a challenge (a question or action required) to the sender of the email.
The message will not be delivered if the challenge is not completed correctly.
DNS failure (bounce_df)
A DNS failure notification indicates that a DNS problem has temporarily prevented email delivery to an address.
Hard Bounce (bounce_hb)
A hard bounce is an email message returned to the sender due to an invalid recipient address.
Hard bounces can happen when the domain name does not exist, for example, if there is a typo in the email (gnail.con rather than gmail.com).
Hard bounces can also happen when the recipient is unknown, resulting in a hard bounce.
Email blocked (bounce_mb)
The email blocked notification indicates that the recipient's email server is blocking the sender's email.
You may see the following messages:
550 Message REFUSED by peer
552 Blocked by filters
Mailbox is full (bounce_fm)
A mailbox is full notification means the recipient's email box is full and is preventing the email server from delivering the message to the recipient.
Bounce none (bounce_none)
Bounce none is a legacy bounce type that is no longer in use. It was used where the bounce was unknown.