A Tokyo-based e-commerce company serving 1.8 million customers faced a deliverability challenge specific to the Japanese market: a significant portion of their customers used email addresses on mobile carrier domains (docomo.ne.jp, ezweb.ne.jp, softbank.ne.jp, i.softbank.jp) and legacy Japanese ISP domains (nifty.com, biglobe.ne.jp, au.com). These domains operate under filtering systems, technical requirements, and relationship protocols that differ substantially from the major global mailbox providers.
The company had been relying on a US-based shared ESP that was optimised for Gmail, Outlook, and Yahoo. Delivery to Japanese carrier email addresses was averaging 52% inbox rate — a significant problem given that 34% of their customer base used carrier email addresses as their primary contact.
Technical environmentJapanese mobile carrier email domains enforce several technical requirements that differ from global standards:
The critical intervention for docomo delivery was registration in the Docomo Postmaster sender registration program — a process requiring documentation of sending practices and volume projections. Unregistered senders are blocked automatically; registration is the minimum requirement for any inbox placement. Once registered, inbox placement at docomo went from near-zero to 87% within 14 days.
For au/EZweb, the conservative connection limit (5 concurrent) enforced by the carrier required queue segmentation so carrier domain messages were routed through a dedicated pool with strictly limited concurrency. Exceeding the connection limit had been causing the carrier to apply 72-hour temporary blocks — visible in the accounting logs as extended deferral sequences.
The infrastructure assessment for this engagement covered four layers: authentication configuration (SPF, DKIM, DMARC alignment), IP reputation status (Postmaster Tools, SNDS, blacklist check), PowerMTA configuration review (domain blocks, throttle settings, bounce handling), and operational practices (list hygiene frequency, bounce processing latency, FBL enrollment and processing status).
Authentication issues were the highest-priority finding. The DKIM key was 1024-bit (below current ISP recommendations of 2048-bit minimum), and DMARC was at p=none with no aggregate reports being collected or reviewed. The combination of outdated authentication and no visibility into sending path failures created an environment where reputation signals were degrading without detection.
The IP pool was rebuilt with traffic type separation as the primary design principle. Transactional traffic (time-sensitive notifications, account events) was assigned a dedicated pool that was never shared with campaign traffic. This separation ensured that campaign performance issues — elevated deferral rates during high-volume sends — could not create queue delays affecting transactional delivery.
| Pool | Traffic Type | IPs | max-smtp-out | Protection Level |
|---|---|---|---|---|
| trans-pool | Transactional notifications | 2 | 10 per IP | Highest — never paused or degraded |
| campaign-pool | Marketing campaigns | 3-4 | 8 per IP | Standard — subject to reputation management |
| warming-pool | New IP warming | As needed | 2-3 per IP | Conservative — warming schedule only |
ISP-specific domain blocks were configured for each major destination: Gmail (max-smtp-out: 8, retry-after: 15m), Outlook (max-smtp-out: 5, retry-after: 20m), Yahoo (max-smtp-out: 6, retry-after: 15m), and ISP-specific configurations for European providers including GMX, Web.de, T-Online, and OVH. Each block included mx-rollup directives to prevent connection count multiplication across MX host variants.
The smtp-pattern-list configuration was extended with custom patterns for ISP-specific diagnostic messages that were not being correctly classified by the default PowerMTA pattern library. These custom patterns ensured that permanent failures (invalid addresses, domain-level blocks) were bounced immediately rather than retried, and that greylisting responses from European ISPs were handled with appropriate retry intervals.
DKIM keys were rotated to 2048-bit RSA on all sending domains. The rotation followed the zero-downtime procedure: publish new public key under new selector, wait 48 hours for DNS propagation, update PowerMTA signing configuration, verify new selector appearing in Authentication-Results headers, then retire old selector after 7 days. DMARC was progressed from p=none through p=quarantine to p=reject over a 12-week period.
Results After 90 DaysThe infrastructure changes produced immediate delivery improvement, but the operational changes — the monitoring discipline and response protocols — are what sustain that improvement over time. Daily Postmaster Tools review and SNDS checks are now part of the infrastructure team's operational routine. FBL reports are processed in real time and feed directly into the suppression system.
The monthly configuration review cycle catches ISP behavior changes before they accumulate into delivery incidents. When Gmail adjusted its bulk sender requirements in 2024, the infrastructure was already operating at the authentication standard required — because the review cycle had identified and addressed the relevant requirements months before the enforcement deadline.
The technical changes in this engagement were straightforward. The more significant work was establishing the monitoring discipline that prevents the gradual drift that caused the original problems — an infrastructure that meets today's ISP requirements but has no ongoing review process will fall behind those requirements within 12-18 months.
— Cloud Server for Email Infrastructure TeamContact the technical team to discuss your specific situation. We assess each environment individually before recommending an architecture.
