Restricting specific users from sending email to external domains in Postfix

Postfix is a free and open-source mail transfer agent (MTA) that routes and delivers electronic mail, intended as an alternative to Sendmail MTA.

When configuring mail server especially for small companies there sometime a requirement to restrict some of the users from sending email to the Internet, and allowing them to send email only to local domain or some domains.

This howto is going to show you how to achieve that extra functionality, this howto is assuming you have working mail server already where by the users are able to send and receive emails.Below are steps,

a. Create a file to catch specific email address which should be restricted.

i. Change to root and open the main Postfix configuration file.

 ~$ sudo su -
 ~# vim /etc/postfix/main.cf

ii. Add rule that will catch limited addresses.

The rule should be added as the first line in smtpd_sender_restrictions as below.

smtpd_sender_restrictions =
check_sender_access hash:/etc/postfix/restricted_senders,

as below:

If you put the rule below any permit rule there possibly it won't work as required.

b. Create specific rule and add the address which should be restricted, am using jarasyola.co.tz domain for the sake of demonstration only, you're going to use you're actual domain in real implementation.

vim /etc/postfix/restricted_senders

Add the email address which should be restricted on above created file and save;

james@jarasyola.co.tz  local_only
vanessa@jarasyola.co.tz local_only

as below

c.i.The rule above says to route restricted emails to a ruleset called "local_only". You need to define it, before you can add rules to it. Open main.conf file and add local_only ruleset as restriction class.

smtpd_restriction_classes = local_only

ii. Then create the restriction class "local_only" and add rules. They are executed top to bottom - first match wins:

local_only=
check_recipient_access hash:/etc/postfix/local_domains,
reject

The two lines should look as below on postfix main.conf configuration file.

d. Lastly, define the local domains where restricted users should be allowed to send, here i allow my local domain only.

Create a file

vim /etc/postfix/local_domains

add the domain(s)

jarasyola.co.tz   OK

If the recipient domain is not in /etc/postfix/local_domains Postfix will look for the next rule.In "local_only" I wrote "reject".This is a static action, which always is true if Postfix tests it. It gives you what you want. Either the recipient domain is on /etc/postfix/local_domains or the request action (send a message) will be rejected.

Drop a comment if you face any difficult in implementing the setup, tuonane kwenye post nyingine.

Configuring Mikrotik router to allow traffic from mail server running on LAN

Its normal to run mail server within the LAN if you have reliable power system but also because it is because is one of the best method for security of your mail server. This how to is going to explain how to configure Mikrotik router to allow traffic going and leaving the mail server which is running in the LAN.

This how to assumes the following

• X.X.X.X being your public IP,
• mail.jarasyola.co.tz being MX records of your server domain and 
• 192.168.1.200 being IP address of mail server.

The setup will have only three simple steps as below.


Step 1
Create firewall to allow inbound TCP traffic from any IP address to these ports on the server.

25 (SMTP)
110 (POP3)
143 (IMAP)
443 (HTTPS)
465 (SMTPS)
587 (Submission)
993 (IMAPS)
995 (POP3S)

I here assume access to webmail and any web access on the server is done on port 443(https) as port 80 will be used by mikrotik web access.

Copy the following on notepad and edit by changing the mail server IP (192.168.1.200) and mikrotik public IP (X.X.X.X)  and paste the following firewall rules on mikrotik terminal.

/ip firewall nat
add action=dst-nat chain=dstnat comment="Email server port forwarding" \
    dst-address=X.X.X.X dst-port=25 protocol=tcp to-addresses=\
    192.168.1.200 to-ports=25
add action=dst-nat chain=dstnat dst-address=X.X.X.X  dst-port=22 \
    protocol=tcp to-addresses=192.168.1.200 to-ports=22
add action=dst-nat chain=dstnat dst-address=X.X.X.X  dst-port=110 \
    protocol=tcp to-addresses=192.168.1.200 to-ports=110
add action=dst-nat chain=dstnat dst-address= dst-port=143 \
    protocol=tcp to-addresses=192.168.1.200 to-ports=143
add action=dst-nat chain=dstnat dst-address=X.X.X.X  dst-port=443 \
    protocol=tcp to-addresses=192.168.1.200 to-ports=443
add action=dst-nat chain=dstnat dst-address=X.X.X.X  dst-port=465 \
    protocol=tcp to-addresses=192.168.1.200 to-ports=465
add action=dst-nat chain=dstnat dst-address=X.X.X.X  dst-port=587 \
    protocol=tcp to-addresses=192.168.1.200 to-ports=587
add action=dst-nat chain=dstnat dst-address=X.X.X.X  dst-port=993 \
    protocol=tcp to-addresses=192.168.1.200 to-ports=993
add action=dst-nat chain=dstnat dst-address=X.X.X.X  dst-port=995 \
    protocol=tcp to-addresses=192.168.1.200 to-ports=995

Step 2
Create DNS redirect to allow creating static DNS entry for the server for local access

Copy and paste the following in the terminal.

/ip firewall nat
add action=redirect chain=dstnat comment="Dns redirect" dst-port=53 protocol=\
    udp to-ports=53
add action=redirect chain=dstnat dst-port=53 protocol=tcp to-ports=53

The setting above will force all dns entry being replied by router before being redirected to provider DNS server.

Step 3
Create the static entry for mail server MX records, so that traffic within the LAN should not be redirected to public IP but going direct to mail server local IP.
Copy and paste the following in mikrotik terminal and change only mail server IP and domain MX records to reflect your mail server.

/ip dns static
add address=192.168.1.200 name=mail.jarasyola.co.tz

That's all, the server will work perfectly within your LAN, to avoid blacklist to spam generated by other users in the network you should block port 25 not being used by other computers in the network for sending unauthorized emails, the howto is found here.

Blocking rogue mail server from LAN using Mikrotik

Rogue mail server is any unauthorized mail server which can be used to send emails usually SPAMS. These are mostly compromised computers in the network which are used by spammers via your public IP's to send SPAM to the rest of the world. The main problem of this is your public IP will be frequently listed in world blacklist database and blocking good operation of legitimate mail servers running within your network.
If you're running mail server inside your LAN its good practice to create a firewall that will allow only your mail server IP to send and receive emails and blocking the rest of IP's.
In mikrotik this can be done by simple 3 firewall lines, the firewall lines are below.

Copy and paste the following on mikrotik terminal.

/ip firewall filter
add chain=forward comment="Allowing mail server" dst-port=25 out-interface=\
    ether1-gateway protocol=tcp src-address=192.168.1.200
add chain=forward comment="Allowing mail server" dst-address=192.168.1.200 \
    dst-port=25 in-interface=ether1-gateway log=yes protocol=tcp
add action=drop chain=forward comment="Blocking Spammers" dst-port=25 log=yes \
    protocol=tcp

The first two lines of the firewall allow traffic going and leaving the mail server where the last line block any traffic leaving the LAN via port 25 which which is mainly used to mail traffic.
In this firewall 192.168.1.200 is mail server IP so if you're copy and pasting that firewall you should change that IP.
After that sit and relax because if you find your public IP being blacklisted is because the spam is originated from your mail server and that is easy to troubleshooting if the server is well configured and secured.