undeliverability

Undeliverability refers to the condition where an email message cannot be successfully delivered to its intended recipient's inbox. This can occur for a variety of reasons, broadly categorized into temporary and permanent issues.

Temporary undeliverability often stems from transient problems. A common cause is a full mailbox on the recipient's server, which prevents new messages from being accepted. Network connectivity issues between the sending and receiving servers can also lead to temporary failures. If the recipient's server is temporarily offline or experiencing high load, delivery might be delayed and subsequently flagged as undeliverable if it exceeds a certain retry period.

Permanent undeliverability indicates that the email address is no longer valid or the recipient's server has permanently rejected the message. The most frequent reason for permanent failure is a non-existent email address, often due to typos in the address, the account being closed, or the domain name no longer being active. Server-side rejections based on content filtering, such as spam detection, or policy violations by the sender can also result in permanent undeliverability. When an email is undeliverable, the sending mail server typically generates a Non-Delivery Report (NDR), also known as a bounce message, which is sent back to the original sender explaining the reason for the failure.