توصية: Proxmox Mail Gateway كطبقة إضافية أمام nextxch

1. Install Proxmox Mail Gateway (ISO, on Debian or as an LXC container)
2. Add relay domains (every domain nextxch accepts)
3. Default relay = nextxch host, port 25 (Mail Proxy -> Relaying)
4. Enable receiver verification, TLS and a certificate in the gateway
5. nextxch: add the gateway IP as a trusted gateway (SpamGuard)
6. Send a test message through the gateway, check Message Trace in nextxch
7. Switch the MX record to the gateway (remove the old MX only after the TTL expires)
8. Firewall: allow port 25 on nextxch from the gateway only
9. Optional outbound: gateway as smart host in nextxch (port 26, STARTTLS)

Example values on this page:
  Gateway:  pmg.company.com   203.0.113.25   (public, MX)
  nextxch:  mail.company.com  10.0.0.20      (internal, mailboxes)

Option A: ISO installer (hardware or VM)
  Download the ISO from proxmox.com/downloads -> boot -> run the installer
  Host name: pmg.company.com   (FQDN, matching the future MX and PTR)
  IP:        static IP, e.g. 203.0.113.25 or a DMZ address behind NAT

Option B: on an existing Debian system
  Add the Proxmox package repository and install the packages as described in
  "Install Proxmox Mail Gateway on Debian" in the PMG admin guide

Option C: LXC container in Proxmox VE
  Proxmox VE -> Storage -> CT Templates -> Templates -> section "mail"
  -> download the Proxmox Mail Gateway template -> create the container

Web interface:  https://pmg.company.com:8006   (port 8006 from the admin network only)

Repositories: without a subscription, add the "pmg-no-subscription" repository.
For production, Proxmox recommends the enterprise repository (with subscription).

Configuration -> Mail Proxy -> Relay Domains
  company.com
  company-group.com        (every domain configured in nextxch, including alias domains)

Configuration -> Mail Proxy -> Relaying
  Default Relay:      mail.company.com  (or 10.0.0.20)
  Relay Port:         25
  Relay Protocol:     SMTP
  Disable MX lookup:  yes               (address the nextxch host directly)

Configuration -> Mail Proxy -> Ports
  External SMTP Port: 25   (from the internet, untrusted)
  Internal SMTP Port: 26   (from nextxch, trusted, for outbound mail)

Configuration -> Mail Proxy -> Options
  Verify Receivers:   550   (reject unknown recipients right away)
  SPF:                yes
  DNSBL Sites:        e.g. zen.spamhaus.org   (observe the list's terms of use)

Several internal servers: Configuration -> Mail Proxy -> Transports
(domain or single address -> its own target server)

; Gateway
pmg.company.com.      3600  IN  A     203.0.113.25
; PTR for 203.0.113.25 at your provider: pmg.company.com

; MX points to the gateway only
company.com.          3600  IN  MX 10 pmg.company.com.

; nextxch stays reachable for clients (IMAP, SMTP submission, webmail)
mail.company.com.     3600  IN  A     198.51.100.20

Test beforehand, without a DNS change:
  swaks --to anna.weber@company.com --server pmg.company.com --port 25 --tls

# A) Global smart host: all outbound mail through the gateway
nextxch-cli update SmtpQueue --set outboundSmartHost=pmg.company.com \
  --set outboundSmartPort=26 --set outboundSmartHostTlsMode=StartTlsRequired

# B) Exception: one partner domain still directly via DNS MX (route without target host)
nextxch-cli create Route --set domain=partner.com --set bypassSmartHost=true \
  --set description="direct via MX, not through the gateway"

# C) Only certain senders through the gateway (#pSmtpQueue -> domain routes)
#    Destination domain:   *
#    Only for senders:     company.com, *.company.com   (SenderDomains)
#    Target host:          pmg.company.com:26           (several, comma-separated = failover)
#    TLS mode:             StartTlsRequired
#    Priority:             50                           (smaller = first; default 100)

# Add the gateway IP to the SPF record of the sender domain:
company.com.  IN  TXT  "v=spf1 mx ip4:203.0.113.25 -all"

# In the gateway: add the nextxch IP under Configuration -> Mail Proxy -> Networks
# if nextxch is not in the same subnet as the gateway.

# nextxch: add the gateway as trusted (public and internal IP, CIDR allowed)
# (admin cockpit: SpamGuard & ClamAV (#pSpam) -> "Trusted gateways")
nextxch-cli update SpamGuard --set trustedRelayCidrs=203.0.113.25,10.0.0.25

# Check: send a test message through the gateway, then in the spam analyzer or Message Trace
#   TRUSTED_RELAY  Via gateway 10.0.0.25, sender IP 198.51.100.7 from Received header
#   SPF_PASS       (SPF now checked against 198.51.100.7)

# Stays active in nextxch:
#   SPF/DKIM/DMARC/ARC, greylisting, DNSBL, adaptive content filter,
#   phishing heuristics, allow/block lists, ClamAV (if enabled)

# Connection limit: all inbound mail comes from one IP.
# Check MaxConnectionsPerIp (#pSmtpQueue, default 48) for high volumes.

Firewall nextxch (inbound)
  TCP 25         only from 203.0.113.25 / 10.0.0.25 (gateway)
  TCP 465, 587   clients (SMTP submission)  -> stay open
  TCP 993, 443   clients (IMAP, webmail, DAV, push) -> stay open

Firewall gateway (inbound)
  TCP 25         from the internet
  TCP 26         from nextxch only (smart host, if used)
  TCP 8006       from the admin network only

TLS internet <-> gateway
  Configuration -> Mail Proxy -> TLS: "Enable TLS" on
  Certificate: Certificates -> ACME (Let's Encrypt) for pmg.company.com

TLS gateway -> nextxch
  nextxch offers STARTTLS on port 25 with the certificate of mail.company.com
  Gateway: TLS destination policy for mail.company.com = encrypt
           (or verify if the default relay is entered as a host name)

TLS nextxch -> gateway (smart host)
  outboundSmartHostTlsMode = StartTlsRequired

DKIM / DMARC
  - nextxch signs outbound mail with DKIM. The gateway forwards it unchanged.
  - No disclaimer rule for outbound mail and no content changes in the gateway
    (otherwise the DKIM signature breaks and DMARC fails at the recipient).
  - Leave DKIM signing off in the gateway; maintain disclaimers in nextxch if needed.

Greylisting
  - Slows down spam bots but delays first messages from new senders (usually < 30 minutes per PMG docs).
  - Run it in the gateway only; behind the gateway it would be ineffective in nextxch.
  - For time-critical senders (e.g. login codes, order confirmations) use the welcomelist,
    or skip greylisting entirely if the other filters are sufficient.

Avoid backscatter
  - Verify Receivers = 550: the gateway asks nextxch via RCPT TO whether the address exists.
  - nextxch rejects unknown recipients at RCPT TO, so the check takes effect immediately.
  - Catch-all addresses in nextxch defeat the check and should be avoided.

Licence:  GNU Affero General Public License v3 (AGPLv3), since PMG 5.0

Optional subscriptions per host and year (net, per proxmox.com, as of 1 Nov 2026):
  Community    EUR 190      no ticket support (forum)
  Basic        EUR 540      5 tickets/year, response 1 business day
  Standard     EUR 1,260    15 tickets/year, response 4 hours
  Premium      EUR 1,900    unlimited tickets, response 2 hours
  (response = first reply to critical requests within business hours)

Sources:
  https://www.proxmox.com/en/products/proxmox-mail-gateway/pricing
  https://pmg.proxmox.com/pmg-docs/pmg-admin-guide.html   (admin guide 9.1.1: licence, repositories, ports)