An email that fails to be delivered to the intended recipient and is returned to the sender by the email server. Bounces can be categorized into two main types:

1. Hard Bounce: Indicates a permanent delivery failure due to issues such as an invalid email address, domain that doesn't exist, or a blocked sender. Hard bounces typically require immediate action to correct or remove the invalid email address from the mailing list.
2. Soft Bounce: Indicates a temporary delivery issue that may be resolved over time, such as a full inbox, a temporary server problem, or a message size that exceeds the recipient's limit. Soft bounces usually require monitoring, and repeated occurrences may necessitate further investigation.

Managing bounce rates is crucial for maintaining a healthy email reputation, ensuring deliverability, and optimizing email marketing efforts.