mount -t tmpfs -o size=2G tmpfs /var/log/pmta This prevents log writes from blocking the SMTP pipeline. Use a cron job to flush logs to persistent storage every hour. Before deploying to production, validate the config:
domain throt-microsoft.com { domain outlook.com domain hotmail.com domain live.com max-smtp-out 40 max-per-host-out 40 max-msg-rate 150/hour warmup { period 10 days start-rate 30/hour end-rate 4000/hour } } sample powermta configuration file hot
domain throt-aol.com { domain aol.com max-smtp-out 20 max-per-host-out 20 } Distribute load across multiple IPs with source hashing. vmta pool-high-volume { source-hash true # Keep same IP per recipient (affinity) max-msg-rate 10000/minute bind-ip-range 192.0.2.10-192.0.2.30 # Your IPs } mount -t tmpfs -o size=2G tmpfs /var/log/pmta This
dkim-signature warmup { domain yourwarmupdomain.com selector feb2025 private-key-file /etc/pmta/dkim/warmup.pem } Route outbound mail based on recipient domain. source { default-domain * use-vmta main-sender domain google.com use-vmta warmup-sender domain yahoo.com use-vmta warmup-sender } ---------- RELAY / NAT LOOPBACK ---------- For internal MUA or API submission. relay { listen 127.0.0.1:2525 auth plain auth-login auth-secret /etc/pmta/auth.secret allow-mail-from *@yourmaindomain.com allow-recipient @ route-to-vmta main-sender } Breaking Down the "Hot" Parameters 1. Connection Tuning ( max-per-host-out , max-smtp-out ) The hottest change from defaults: max-per-host-out 100 . Standard config uses 20. Bumping to 100 allows simultaneous delivery to large ISPs like Gmail. However, never exceed 200—Google’s postmaster guidelines suggest 100-120 per IP. 2. Warmup Schedules (Built-in) The warmup { } block inside domain directives is a game-changer. Instead of manually raising limits, PowerMTA automatically increases max-msg-rate over 7-10 days. This prevents ISPs from throttling you as a "sudden burst" sender. 3. Virtual MTA Pools with Source Hashing source-hash true ensures the same recipient domain always uses the same source IP. This is critical for maintaining reputation. Without hashing, rotating IPs randomly can cause ISPs to see "new" IPs for known recipients, triggering spam filters. 4. Logging to RAM Disk (Hidden Hot Trick) The config references /var/log/pmta . In a truly hot system, you should: vmta pool-high-volume { source-hash true # Keep same
route backup-dc { destination 203.0.113.5:2525 backup true priority 10 } Before sending real campaigns, use:
Introduction: What Makes a PowerMTA Config "Hot"? In the world of high-volume email delivery, PowerMTA (PMTA) by SparkPost is the gold standard for MTA (Mail Transfer Agent) software. However, a standard, out-of-the-box configuration will not survive a modern sending environment. To handle millions of emails per hour, you need a "hot" configuration file —one that is aggressively tuned for throughput, DKIM signing, bounce handling, and ISP throttling.
/usr/sbin/pmta check config /etc/pmta/config Then, stress-test with a small warmup list: