Postfix and BillionMail

Workflow

1. Determine the active SMTP stack from listening sockets and container mappings before reading configuration. Do not assume a dormant host Postfix package is authoritative.
2. Inspect the active container/runtime and mounts to discover effective configuration, persistent configuration, spool and mail-log paths. Treat observed mounts as evidence; do not hard-code paths from another installation.
3. Capture postconf -n or the effective equivalent from the active instance before proposing changes.
4. Trace messages by queue ID through Postfix and filtering/signing components. Separate unique queue IDs from retry attempts.
5. Inspect current queue state and oldest deferred items before flush/requeue/delete operations.
6. Inspect relayhost, transport_maps, sender-dependent transports, myhostname, mydestination, networks, SASL, TLS, DNS dependencies and inet_protocols where relevant.
7. Inspect Rspamd/DKIM signing configuration for selector/domain duplication and verify corresponding DNS.
8. For containerized BillionMail, inspect persistent host mounts before relying on host journalctl; authoritative mail logs may live in a mounted container log directory instead of the host journal.
9. Make durable changes in BillionMail's persistent/generated configuration source, not only inside the running container. Verify behavior after reload and, when safe, after container recreation.
10. For destination-specific timeout/refusal clusters, hand off classification and control testing to smtp-production-debugging. For deterministic alternate routing, use postfix-selective-relay.

Diagnostic principles

• A deferred queue entry is not automatically a new failed message; retries can generate many log lines for one queue ID.
• A TCP timeout before SMTP banner is technically different from an SMTP 4xx/5xx response.
• Successful delivery to one provider does not prove reachability to every destination.
• A transient failure against one MX/IP can still end in successful delivery through another MX; inspect final queue outcome.
• If IPv6 is enabled for outbound SMTP, verify equivalent PTR, SPF, identity and deliverability assumptions before treating IPv4 evidence as complete.