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Contributed by Dominic Yip and Enrico Werner, Cisco TAC Engineers.
What happens to incoming messages in the event of a system failure on an ESA?
If your ESA fails to respond due to loss of connectivity or system failure, your inbound emails will receive a soft bounce (400-level SMTP error) and the sending MTA (Message Transfer Agent) will retry sending emails based on its bounce profile.
If you configured a secondary MX record, the retry will be directed to the host specified in the secondary MX record. The SMTP standard suggest to try to retry for 4 days, most MTAs follow this guideline so even a longer connection problem should not result in a lose of emails.