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Set up DKIM, SPF and DMARC for Postfix on Debian 13 with opendkim, selector rotation, a staged DMARC rollout and checks with swaks and opendkim-testkey.

How SMTP smuggling abuses bare-LF end-of-data handling to pass SPF, and how to detect and block it in Postfix on Debian 13 with a self-test.

Fix Postfix Relay access denied, deferred port 25 timeouts, Domain not found and SASL failures on Debian 13, with diagnostic commands and an open-relay test.