The audit covers the eight dimensions where SMTP server performance actually lives. Default Linux kernel ships with conservative TCP buffer values and connection-handling parameters tuned to work reliably across hardware ranging from a Raspberry Pi to a 32-core EPYC server. Those defaults are not optimal for a server handling 50,000 concurrent SMTP connections sustained, which is the load most B2B platforms hit during scheduled campaign windows. Default PowerMTA configuration ships with virtual-mta concurrency caps and retry-interval ladders calibrated for safety, not throughput. The audit identifies which conservative defaults are costing throughput in your specific workload and proposes calibrated values, with explicit rollback procedures documented for each change. No magic, no proprietary scoring, no black boxes that your team cannot inspect after we leave.
DIMENSION 1 · KERNEL SYSCTL
Network stack and connection handling
TCP buffer sizing for the workload, FIN timeout reduction for short-lived SMTP connections, ephemeral port range expansion for outbound concurrency, listen backlog depth aligned to peak connection rate, and BBR congestion control for production where Cloudflare and Google data validate the performance gains.
Tuned: tcp_rmem · tcp_wmem · tcp_fin_timeout · ip_local_port_range · somaxconn · netdev_max_backlog · tcp_congestion_control · tcp_tw_reuse · tcp_no_metrics_save
DIMENSION 2 · CPU & NUMA
CPU governor, IRQ affinity, NUMA topology
Performance governor pinned (no race-to-idle penalties), IRQ affinity aligned to NIC NUMA node, RPS and RFS for receive-side scaling on multi-queue NICs, kernel.sched_migration_cost_ns tuned to reduce gratuitous task migrations, and transparent huge pages disabled if they cause latency spikes during memory reclaim.
Tuned: cpu_governor performance · irqbalance config · /proc/irq/N/smp_affinity · RPS/RFS · sched_migration_cost_ns · transparent_hugepage · numactl pinning · taskset for PowerMTA processes
DIMENSION 3 · MTA-SPECIFIC
PowerMTA, Halon, MailerQ, Postfix concurrency
Per-domain and per-ISP concurrency calibrated to the throttle policies of Gmail, Yahoo, Microsoft, Apple as they stand in 2026, retry-interval ladders that respect 4XX soft-fail backoff without abandoning legitimate retries, queue-buckets distribution to prevent head-of-line blocking, virtual-mta or VirtualMTA pool design aligned to IP reputation tiers.
PowerMTA: max-msg-per-connection · max-smtp-out · retry-after · queue-buckets · virtual-mta concurrency
Halon HSL: queue policies · smtpd profiles
Postfix: smtp_destination_concurrency_limit · default_destination_rate_delay
DIMENSION 4 · TCP STACK
Congestion control, MSS, slow-start behavior
BBR or BBRv2 congestion control where kernel supports it (default cubic is suboptimal for short-lived high-throughput SMTP connections), slow_start_after_idle disabled because every SMTP connection becomes idle between batches and slow start kills throughput, MSS clamping tuned for VPN-tunneled deployments, and tcp_no_metrics_save to prevent stale metrics polluting new connections.
Tuned: net.ipv4.tcp_congestion_control=bbr · net.ipv4.tcp_slow_start_after_idle=0 · net.ipv4.tcp_mtu_probing=1 · net.ipv4.tcp_no_metrics_save=1 · net.core.default_qdisc=fq
DIMENSION 5 · I/O & FILESYSTEM
Mount options, scheduler, fsync behavior
I/O scheduler tuned per device class (none for NVMe, mq-deadline for spinning rust if any remain), mount options aligned to workload (noatime essential, nodiratime, lazytime where supported), fsync behavior on PowerMTA accounting logs balanced between durability requirement and throughput cost, block size alignment to underlying storage geometry.
Tuned: /sys/block/dev/queue/scheduler · mount opts noatime,nodiratime,lazytime · vm.dirty_ratio · vm.dirty_background_ratio · accounting log fsync policy · log rotation interval
DIMENSION 6 · OBSERVABILITY
Metrics, logs, alert calibration
Prometheus exporters covering kernel metrics, MTA-specific accounting, and TCP stack health. Loki or ELK pipeline for accounting log shipping with parsing rules tuned to extract bounce reason, mailbox provider latency, queue depth distribution. Alert thresholds calibrated against the post-tuning baseline so they catch regressions without firing on noise during normal operation.
Stack: node_exporter · powermta_exporter · custom MTA accounting parser · Grafana dashboards (8 panels) · Loki log shipping · alert thresholds (12 rules calibrated to baseline)
DIMENSION 7 · DNS RESOLUTION
Local resolver, MX lookup, recursive redundancy
Local resolver configured with aggressive caching (Unbound or systemd-resolved with appropriate cache size), MX lookup profiling to detect slow recursive paths, secondary recursive resolver configured for failover (because the day Google Public DNS has a regional issue, your bounce rate spikes if you depend on a single resolver), MX prefetching for known high-volume destinations during off-peak.
Tuned: Unbound or systemd-resolved cache size · upstream resolver redundancy (Google + Cloudflare + local) · MX prefetch cron · DNS-over-TLS where MTA supports it · resolv.conf options (timeout, attempts)
DIMENSION 8 · TLS HANDSHAKE
Session reuse, OCSP stapling, cipher ordering
TLS 1.2 minimum with TLS 1.3 preferred per Microsoft 2026 enforcement, session ticket reuse to amortize handshake CPU cost across many connections, OCSP stapling to remove a synchronous round-trip during certificate validation, cipher suite ordering aligned to Mozilla intermediate profile, DANE/TLSA validation if the client publishes records.
Tuned: TLS protocol minimum 1.2 · session_ticket_reuse on · OCSP stapling · cipher suite Mozilla intermediate · DANE/TLSA validation · cert renewal monitoring (no rotation logic touched)