In reality despite ConsumeIMAP is stateless, the IMAP server holds /Recent flag that is system read-only flag (theoretically within IMAP session, but how Gmail detect unique session is a BlackBox and I was not able to manage it with NiFi ConsumeIMAP client). If you want to re-fetch all your historical emails I recommend using ConsumePOP3 NiFi component, with the following setting on Gmail server: Gmail - settings - all - forwarding - Enable POP for all mail After performing such historical fetch you can disable ConsumePOP3 and continue to work with ConsumeIMAP to consume new messages.
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