What is the default value of retry attempts before a message will hard bounce?

The default value of retry attempts before a message will hard bounce is five. This means that when an email sent through the Mimecast system encounters a delivery failure, the system will attempt to resend the email up to five times before determining that the message cannot be delivered. A hard bounce indicates a permanent failure, typically because the recipient's email address is invalid, either due to being misspelled or because the mailbox no longer exists.

The reasoning for setting this threshold at five attempts is to provide a reasonable balance between persistently attempting to deliver important messages while also promptly acknowledging that delivery will not occur if repeated attempts fail. This approach helps to manage both system resources and the sender's expectations regarding the status of their sent messages. After five failed attempts, the system stops trying to send the message, and a hard bounce notification is generated for the sender's awareness.