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No manual channel choice or resend is needed. Retries, timeouts and unconfirmed results do not switch channels, so customers do not receive two copies. Operators may still send manually, with failures from the last 30 days shown for review first.
When SPF, DKIM and DMARC all fail on an inbound message, the system does not send an automatic acknowledgement. An operator can still reply after verifying the sender.
Applies only to mailboxes with the backup channel enabled and only to emails sent after it is enabled. Historical emails are never resent.