High-Availability Outbound Email: Postfix smtp_fallback_relay with HAProxy SMTP Balancing

For organizations operating mail infrastructure in restrictive networks or corporate subnets with unreliable direct port 25 connectivity, combining HAProxy as an SMTP proxy relay with Postfix's smtp_fallback_relay directive guarantees reliable email delivery without dropped messages.

Flow: Primary MTA → Direct MX Delivery Failure → smtp_fallback_relay → HAProxy Relay VPS → Destination MX

1. HAProxy TCP Relay Configuration (haproxy.cfg)

listen smtp-relay
    bind *:2525
    mode tcp
    option tcplog
    timeout connect 5s
    timeout client 1m
    timeout server 1m
    server mta-out 10.0.0.1:25 check

2. Postfix Fallback Configuration (/etc/postfix/main.cf)

Instruct Postfix to attempt direct MX delivery first, but automatically route through the HAProxy relay if destination mail servers are unreachable:

# Automatic fallback routing to HAProxy relay on port 2525
smtp_fallback_relay = [relay.example.com]:2525