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Note that on certain failures, the retry will NOT happen even when it's configured this way. This could happen when the failure that occured is considered FATAL (for example: a 401 HTTP response code which means that there's a problem with permissions). The reason for this is that even when an retry is configured, the message will still fail the second time arround, therefore no retry will happen.
Note: Retryable deliver/notification is only available in version AS4.NET v.3.1.0 and beyond.