What are invalid addresses?

Invalid addresses do not follow the structural rules of RFC 5322 or do not correspond to a real mailbox. Examples include addresses missing an at sign, domains that do not exist, or domains without MX records.

Addresses with syntactic problems such as double periods in the domain or mismatched quoting are also invalid. Domains that fail DNS resolution return an NXDOMAIN code which leads to a hard bounce.

Sending to invalid addresses is the primary cause of high hard bounce rates. Mailbox providers expect this rate to remain below 0.5%. Exceeding that threshold signals poor hygiene and harms sender reputation.

An invalid address is a ship with broken coordinates that can never reach a harbor.