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Outbound Transports and MTA

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Tyler Hatfield edited this page Jul 13, 2026 · 1 revision

Outbound Transports & MTA

PolyPress supports hybrid outbound sending strategies, combining high-speed local MTA direct deliveries with tenant-level relays and host-wide SMTP fallbacks.

🚀 Direct Send Mode (Internal MTA)

In Direct Send Mode, PolyPress behaves as a full Mail Transfer Agent (MTA). It queries DNS servers directly for the MX records of recipient email domains, opens SMTP connections to destination servers on port 25, and delivers messages without using third-party relays.

1. Reverse PTR (rPTR) & Egress IPs

Direct delivery requires proper DNS setups to prevent spam filters from rejecting your emails:

  • Ensure your sending server's reverse DNS (rPTR) record matches the domain configured in the Mail Server HELO/EHLO Identity settings.
  • If your host uses multiple IP interfaces, configure the Sending IP Override to force outbound connections through the correct IP.

2. DKIM Signature Generation

MTA direct deliveries are signed with 2048-bit RSA key pairs:

  1. Navigate to Sending Settings -> Direct Send.
  2. Enable DKIM generation to output the public key.
  3. Add the corresponding TXT record to your domain's DNS registry (typically using host prefix polypress._domainkey).

⚙️ Tenant-Level SMTP Relays

If port 25 is blocked by your hosting provider, you can configure external SMTP servers (e.g. Resend, Mailgun, Postmark, AWS SES, or SendGrid) on ports 587 or 465 for individual tenants:

  • Configure the SMTP Host name, Port, Username, and Password.
  • Toggle connection security preferences (Use SSL or Use TLS/STARTTLS).

🛡️ Global SMTP Relay Fallback

Super Admins can define a host-wide fallback SMTP relay in the Developer Console:

  • If a tenant does not have a local SMTP configuration and attempts to dispatch transactional emails or newsletters, the background queue worker automatically redirects the traffic via this global relay.
  • This ensures transactional delivery is maintained even during tenant onboarding.

🧵 Thread Allocation & Rate Limits

Outbox queue dispatching is managed by multithreaded task workers:

  • Max Concurrent Sending Threads: Configure thread limits per tenant workspace to allocate connection limits across your mail server.
  • Hourly Rate Limiting: Limit hourly throughput speeds per tenant (e.g., max 500 emails/hour) to comply with external relay rate limits.
  • Diagnostics: View diagnostics logs showing live thread usage counts, hourly outbox throughput charts, and transient failure status messages.

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