Private email infrastructure & deliverability engineering
northboundmail.net
NorthboundMail Systems
Est. 2011 — SMTP infrastructure & deliverability
Infrastructure

SMTP Relay

Authenticated SMTP submission to a hardened relay cluster, with per-stream credentials, envelope shaping and full log access. Suitable for senders migrating off shared platforms without giving up control of their DNS or content.

Overview

Our SMTP relay is a submission endpoint backed by clustered MTAs that we operate directly. Clients connect over TLS with per-stream credentials and hand off messages by SMTP or SUBMISSION. The relay accepts, authenticates, aligns envelopes, signs with the client's DKIM key, routes to the appropriate outbound IP pool and processes bounces and complaints back to the client.

Unlike a generic transactional API, the relay is designed to preserve control at the sender's side: the client keeps their sending domains, DKIM keys and content templates. Northbound provides the outbound reputation, IP posture and deliverability engineering behind the endpoint.

Every relay account is issued with rate limits, per-stream tagging and complaint routing that reflect the client's actual sending profile — not a shared template.

Benefits

  • Migration off a shared platform without rewriting your application
  • Per-stream credentials — transactional, marketing and internal traffic isolated
  • Full envelope, header and bounce logs available to the sender
  • TLS 1.2/1.3 required; certificate pinning available for high-security integrations
  • Rate control and queue policy tuned to the sender's traffic pattern

Technical specifications

ProtocolsSMTP (25), Submission (587), SMTPS (465)
AuthenticationSASL PLAIN, LOGIN, CRAM-MD5, XOAUTH2
Transport securitySTARTTLS mandatory; TLS 1.2 minimum
ThroughputUp to 250 msg/sec per credential (higher on request)
Message size50 MB default; 100 MB on request
Log retention60 days hot, 12 months archived
SLA99.95% acceptance uptime, monitored externally

Use cases

  • — SaaS application that has outgrown its transactional ESP but does not want a full self-hosted MTA
  • — Marketing team that needs to move off a shared IP pool without changing sending platform
  • — Agencies that need one submission endpoint with per-client tagging
  • — Financial and health senders that require log retention beyond typical ESP defaults

Questions

Do you accept any content over the relay?
No. All accounts operate under our Acceptable Use Policy. Content that violates local law, is deceptive, or would materially damage the sending IPs is rejected. We do not run open relays.
Can I bring my own DKIM keys?
Yes. We can generate keys for you or import existing keys. Rotation is scheduled or on-demand.
Is the relay multi-region?
Yes. Submission endpoints are available in EU, US and APAC regions with region-affinity routing on request.

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