When a receiving mail server returns a temporary SMTP response code (4xx), Email Gateway Defense (EGD) marks the mail as deferred. This deferral is returned back to the sending server. Mail that is deferred for any reason is not delivered by EGD. The sending server is responsible for retrying to send the mail.
Barracuda Networks recommends the following to ensure your mail is delivered in a timely manner:
- Recipient limit – The maximum number of recipients in a single email message. While there is no recipient limit from EGD, EGD will process the first 100 recipients and defer the rest. The sending server will then try to send the message to the next 100 recipients. This will continue until the message has been sent to all valid recipients. Barracuda Networks recommends splitting the message into "chunks" of fewer than 100 recipients. Each "chunk" is effectively treated as a new message.
- Message deferral limit – The amount of time to wait before a message is sent again to the recipient. Barracuda Networks recommends setting the message retry interval to 15 minutes on the sending mail server. Message retry attempts are based on the error type received from the recipient's mail system.