Skip to content

Relay inbound and outbound mail for an internal DMS

Introduction

Community contributed guide

Adapted into a guide from this discussion.

Requirements:

  • A public server with a static IP, like many VPS providers offer. It will only relay mail to DMS, no mail is stored on this system.
  • A private server (e.g.: a local system at home) that will run DMS.
  • Both servers are connected to the same network via a VPN (optional convenience for trust via the mynetworks setting).

The guide below will assume the VPN is setup on 192.168.2.0/24 with:

  • The public server is using 192.168.2.2
  • The private server is using 192.168.2.3

The goal of this guide is to configure a public server that can receive inbound mail and relay that over to DMS on a private server, which can likewise submit mail outbound through a public server or service.

The primary motivation is to keep your mail storage private instead of storing it to disk unencrypted on a VPS host.

DNS setup

Follow our standard guidance for DNS setup.

Set your A, MX and PTR records for the public server as if it were running DMS.

DNS Zone file example

For this guide, we assume DNS is configured with:

  • A public reachable IP address of 11.22.33.44
  • Mail for @example.com addresses must have an MX record pointing to mail.example.com.
  • An A record for mail.example.com pointing to the IP address of your public server.
$ORIGIN example.com
@     IN  A      11.22.33.44
mail  IN  A      11.22.33.44

; mail server for example.com
@     IN  MX  10 mail.example.com.

SPF records should also be set up as you normally would for mail.example.com.

Public Server (Basic Postfix setup)

You will need to install Postfix on your public server. The functionality that is needed for this setup is not yet implemented in DMS, so a vanilla Postfix will probably be easier to work with, especially since this server will only be used as an inbound and outbound relay.

It's necessary to adjust some settings afterwards.

Create or replace /etc/postfix/main.cf 
# See /usr/share/postfix/main.cf.dist for a commented, more complete version

smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
biff = no

# appending .domain is the MUA's job.
append_dot_mydomain = no

# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h

# See http://www.postfix.org/COMPATIBILITY_README.html -- default to 3.6 on
# fresh installs.
compatibility_level = 3.6

# TLS parameters
smtpd_tls_cert_file=/etc/postfix/certificates/mail.example.com.crt
smtpd_tls_key_file=/etc/postfix/certificates/mail.example.com.key
smtpd_tls_security_level=may
smtp_tls_CApath=/etc/ssl/certs
smtp_tls_security_level=may
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache

alias_database = hash:/etc/aliases
alias_maps = hash:/etc/aliases
maillog_file = /var/log/postfix.log
mailbox_size_limit = 0
inet_interfaces = all
inet_protocols = ipv4
readme_directory = no
recipient_delimiter = +

# Customizations relevant to this guide:
myhostname = mail.example.com
myorigin = example.com
mydestination = localhost
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128 192.168.2.0/24
smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
transport_maps = hash:/etc/postfix/transport
relay_domains = $mydestination, hash:/etc/postfix/relay

# Disable local system accounts and delivery:
local_recipient_maps =
local_transport = error:local mail delivery is disabled

Let's highlight some of the important parts:

  • Avoid including mail.example.com in mydestination, in fact you can just set localhost or nothing at all here as we want all mail to be relayed to our private server (DMS).
  • mynetworks should contain your VPN network (eg: 192.168.2.0/24 subnet).
  • Important are transport_maps = hash:/etc/postfix/transport and relay_domains = $mydestination, hash:/etc/postfix/relay, with their file contents covered below.
  • For good measure, also disable local_recipient_maps.
  • You should have a valid certificate configured for mail.example.com.

Open relay

Please be aware that setting mynetworks to a public CIDR will leave you with an open relay. Only set it to the CIDR of your VPN beyond the localhost ranges.

When mail arrives to the public server for an @example.com address, we want to send it via the relay transport to our private server over port 25 for delivery to DMS.

transport_maps is configured with a transport table file that matches recipient addresses and assigns a non-default transport. This setting has priority over relay_transport.

Create /etc/postfix/transport

example.com relay:[192.168.2.3]:25

Other considerations:

  • If you have multiple domains, you can add them here too (on separate lines).
  • If you use a smarthost add * relay:[X.X.X.X]:port to the bottom (eg: * relay:[relay1.org]:587), which will relay everything outbound via this relay host.

Tip

Instead of a file, you could alternatively configure main.cf with transport_maps = inline:{ example.com=relay:[192.168.2.3]:25 }

We want example.com to be relayed inbound and everything else relayed outbound.

relay_domains is configured with a file with a list of domains that should be relayed (one per line), the 2nd value is required but can be anything.

Create /etc/postfix/relay

example.com   OK

Tip

Instead of a file, you could alternatively configure main.cf with relay_domains = example.com.

Files configured with hash: table type must run postmap to apply changes

Run postmap /etc/postfix/transport and postmap /etc/postfix/relay after creating or updating either of these files, this processes them into a separate file for Postfix to use.

Private Server (Running DMS)

You can set up your DMS instance as you normally would.

  • Be careful not to give it a hostname of mail.example.com. Instead, use internal-mail.example.com or something similar.
  • DKIM can be setup as usual since it considers checks whether the message body has been tampered with, which our public relay doesn't do. Set DKIM up for mail.example.com.

Next, we need to configure our private server to relay all outbound mail through the public server (or a separate smarthost service). The setup is similar to the default relay setup.

Create postfix-relaymap.cf

@example.com  [192.168.2.2]:25

Meaning all mail sent outbound from @example.com addresses will be relayed through the public server at that VPN IP.

The public server mynetworks setting from earlier trusts any mail received on port 25 from the VPN network, which is what allows the mail to be sent outbound when it'd otherwise be denied.

Create postfix-main.cf

mynetworks = 192.168.2.0/24

This will trust any connection from the VPN network to DMS, such as from the public server when relaying mail over to DMS at the private server.

This step is necessary to skip some security measures that DMS normally checks for, like verifying DNS records like SPF are valid. As the mail is being relayed, those checks would fail otherwise as the IP of your public server would not be authorized to send mail on behalf of the sender address in mail being relayed.

Alternative to mynetworks setting

Instead of trusting connections by their IP with the mynetworks setting, those same security measures can be skipped for any authenticated deliveries to DMS over port 587 instead.

This is a bit more work. mynetworks on the public server main.cf Postfix config is for trusting DMS when it sends mail from the private server, thus you'll need to have that public Postfix service configured with a login account that DMS can use.

On the private server, DMS needs to know the credentials for that login account, that is handled with postfix-sasl-password.cf:

@example.com user:secret

You could also relay mail through SendGrid, AWS SES or similar instead of the public server you're running to receive mail from. Login credentials for those relay services are provided via the same postfix-sasl-password.cf file.

Likewise for the public server to send mail to DMS, it would need to be configured to relay mail with credentials too, removing the need for mynetworks on the DMS postfix-main.cf config.

The extra effort to require authentication instead of blind trust of your private subnet can be beneficial at reducing the impact of a compromised system or service on that network that wasn't expected to be permitted to send mail.

IMAP / POP3

IMAP and POP3 need to point towards your private server, since that is where the mailboxes are located, which means you need to have a way for your MUA to connect to it.