What is a NDR (Non delivery report)?
In the Internet's standard e-mail protocol SMTP, a bounce message, also called a Non-Delivery Report/Receipt (NDR), a (failed) Delivery Status Notification (DSN) message, a Non-Delivery Notification (NDN) or simply a bounce, is an automated electronic mail message from a mail system informing the sender of another message about a delivery problem. The original message is said to have bounced.
NDR's will typically contain information about any SMTP Error codes in the email, and error codes will generally be of the form 5.x.x or 4.x.x
The messages themselves can be identified by having their sending email-address set as 'empty' - shown as '<>' in the logs.
Out-of-office replies are also considered as NDRs.
Outmail does not, by default, support sending of NDRs. This is because many email servers are configured to send 'Bounce' NDRs when they should actually not accept the email to begin with. This is the main cause of back-scatter spam on the Internet. Please contact us if you require support for outbound NDR/OOO - see this article for more information