Relaying outgoing emails through Postfix

Relaying

Your mail server is almost ready for use. But one puzzle piece is missing. Your users can receive emails but they cannot send them yet.

Please note that there is a difference on how users send emails versus how servers send emails. For comparison:

• A mail server fetches the MX record for the domain name of the recipient’s email address. That tells it which mail server to talk to. Then it opens an SMTP connection (TCP port 25) and sends the email.
• An end user with a mail client like Thunderbird, Evolution or Mutt cannot do it this way. The mail clients have no functionality built in for that MX record fetching magic. And the user is most likely on a dynamic IP address that other mail servers do not trust and reject. End users are meant to send emails to their provider’s (your!) mail server, send login information to authenticate themselves and then send the email. This is called relaying because your mail server acts as a relay between the user and other mail servers on the internet. For security reasons the user also has to authenticate to be allowed to send emails.

I created a couple of illustrations to explain it.

Incoming email

When someone on the internet sends an email to john@example.org, some other mail server will deliver the email using SMTP to your mail server. Postfix will determine that it’s responsible for email addresses in the example.org domain and accept the email. John can then get the email from your server.

Outgoing email (without authentication)

John is on the internet somewhere and wants to send an email to lisa@example.com. Your mail server is not responsible for the “example.com” domain so it receives John’s email and would have to forward (relay) it to the actual mail server that is responsible for …@example.com email addresses. This may seem like a harmless scenario but your mail server must deny that:

Why? Because anyone can claim to be John and make your mail server forward mail. If an attacker (like a spammer) would send millions of spam emails in John’s name through your server then other organisations will accuse you as the operator of the mail server of spamming. Your mail server would be what people call an open relay. That is not what you want because your mail server would get blacklisted and you will not be able to send out mail to most other servers. So without any proof that the sender is indeed John, your server must reject the email.

Outgoing email (with authentication)

So how does John prove his identity? He needs to use authenticated SMTP. This is similar to the previous case but John’s email program will also send his username and password.

We are making sure that his authentication happens over an encrypted connection so John’s password is safe.

Postfix setting “mynetworks”

In addition to using SMTP authentication you can tell Postfix to always allow relaying for certain IP addresses. The mynetworks setting contains the list of IP networks or IP addresses that you trust. Usually you define your own local network here. The reason John had to authenticate in the above example is because his IP address is not part of mynetworks.

Make Postfix use Dovecot for authentication

Enabling SMTP authentication in Postfix is surprisingly easy. You already configured Dovecot so it knows all about your users from the SQL database. So let’s just make Postfix use that by telling it to ask the Dovecot server to verify the username and password. Postfix just needs some extra configuration. Run these commands on the shell:

postconf smtpd_sasl_type=dovecot
postconf smtpd_sasl_path=private/auth
postconf smtpd_sasl_auth_enable=yes

This enables SMTP authentication and tells Postfix that it can talk to Dovecot through a socket file located at /var/spool/postfix/private/auth. Do you remember that Postfix runs in a sandboxed chroot directory? That’s at /var/spool/postfix. It cannot access any files outside of that directory. But fortunately in a previous section you edited the /etc/dovecot/conf.d/10-master.conf file and made Dovecot place a socket file into /var/spool/postfix/private/auth to allow communication from Postfix.

Enable encryption

The following settings enable encryption, set the key and certificate paths for Postfix. Just run these commands:

postconf smtpd_tls_security_level=may
postconf smtpd_tls_auth_only=yes
postconf smtpd_tls_cert_file=/etc/letsencrypt/live/webmail.example.org/fullchain.pem
postconf smtpd_tls_key_file=/etc/letsencrypt/live/webmail.example.org/privkey.pem
postconf smtp_tls_security_level=may

The above settings allow encrypted incoming (smtpd_) and outgoing (smtp_) connections. But they do not enforce it. So if a remote mail server does not have encryption enabled we will still accept their emails. You may be tempted to enforce encryption but that would reject email communication with servers who have been configured without encryption.

However the smtpd_tls_auth_only=yes setting makes sure that the user’s authentication information (email address and password) are always encrypted between the user and your mail server.

How SMTP authentication works

Are you curious how SMTP authentication looks on a low level? You probably are not. But let’s do it anyway. Just once, so that you get the idea.

In previous versions of this guide we used “telnet” to connect to TCP port 25 and speak SMTP. But now we enforce encryption and can’t do SMTP authentication unencrypted. Let’s try it anyway:

telnet localhost smtp

The server will let you in:

Trying 127.0.0.1…
Connected to localhost.
Escape character is '^]'.
220 webmail ESMTP Postfix (Debian/GNU)

Say hello:

ehlo example.com

Postfix will present a list of features that are available for you:

250-mailtest
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-DSN
250-SMTPUTF8
250 CHUNKING

Let me briefly explain what these lines mean:

• PIPELINING
  This is a feature to speed up SMTP communication. Usually the remote system has to wait for a response to every command it sends. Pipelining allows the remote server to send several commands in a batch without waiting for a response. Postfix will just store these commands and execute them one by one. If you told Postfix to forbid pipelining it would disconnect the remote server when it tries to send bulks of commands without waiting for the proper reply. It is mainly a feature against rogue senders.
• SIZE 10240000
  The remote server is allowed to send emails up to 10 MB large. This has long been a common maximum size for emails. However nowadays 40 MB or even more are more common sizes because emails have grown larger. Some mail servers even allow 100 MB.
• VRFY
  Allows remote servers to verify a given name or email address. For example the remote server could send “VRFY john” and your server might respond “250 John Doe <john@example.org>”. It can be used to verify that a certain recipient email address is deliverable
• ETRN
  A command that a remote system can send to flush the Postfix queue of mails for a certain domain. It can be used if the remote system had technical problems and failed to receive email for a while. Then it could send an ETRN command to make your server start sending outstanding emails for that domain. It is rarely used.
• STARTTLS
  This tells the remote system that it might start switching from this unencrypted to an encrypted connection by sending the “STARTTLS” command. It will then start negotiating a TLS-encrypted connection. You could compare it to an HTTP connection that suddenly switches over to an encrypted HTTPS connection. The advantage is that you can start talking SMTP on TCP port 25 and don’t have to open up a second TCP port like 465 which is the “SSMTP” (secure SMTP) port and only accepts encrypted connections.
• ENHANCEDSTATUSCODES
  This enables more three-digit return codes for various conditions. See the RFC2034 if you are curious.
• 8BITMIME
  In ancient times SMTP only processed 7-bit characters. You couldn’t transfer special characters like “Ä” or “ß” without special encoding. 8BITMIME allows a transmission of emails using 8-bit characters. Still many emails are specially encoded using ISO8859-1 or UTF-8.
• DSN
  It enables DSNs (delivery status notofications) that allows the sender to control the messages that Postfix creates when an email could not be delivered as intended.
• SMTPUTF8
  In addition to 8BITMIME you can use UTF8 encoded characters in header fields.
• CHUNKING
  This feature (described in RFC 3030) makes sending of large emails more efficient.

However one important line is missing here that would allow us to send our username and password:

250-AUTH PLAIN LOGIN

We told Postfix to only allow authentication when the connection is encrypted. So we are not offered authentication over this plain text connection. That’s what we want.

Are you still connected? Okay, good. So we need an encrypted connection using TLS. Using the STARTTLS feature we can switch over from unencrypted to encrypted without having to reconnect. Enter…

STARTTLS

And the server replies:

220 2.0.0 Ready to start TLS

However now it’s getting weird because you would have to speak TLS encryption which is not a language that humans speak. So let’s quit this using the “QUIT” command and do that differently.

We can use OpenSSL to help us with the decryption. Run:

openssl s_client -connect localhost:25 -starttls smtp

You will see a lot of output. OpenSSL has connected to TCP port 25 and issued a STARTTLS command to switch to an encrypted connection. So whatever you type now will get encrypted. Enter:

EHLO example.com

And Postfix will send a list of capabilities that will look like this:

250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-AUTH PLAIN LOGIN
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DSN

And now that we are using an encrypted connection Postfix offers us to authenticate. So let us send the authentication string with a Base64-encoded password:

AUTH PLAIN AGpvaG5AZXhhbXBsZS5vcmcAc3VtbWVyc3Vu

Unless you have changed John’s password to something else than “summersun” the server should accept that authentication:

235 2.7.0 Authentication successful

Excellent. You are logged in through SMTP. You could now send an email to be forwarded to another mail server. I just wanted to show that authentication works if you use an encrypted connection.

Disconnect from Postfix:

QUIT

Authentication works. Well done.

Base64-encoded passwords

Wait a minute. What was that crazy cryptic string? There was no username and password in it. Was it encrypted?

No, that was no encryption. It was merely an encoded version of the username and password. Why that? Well usually in SMTP you can only transfer ASCII characters. But the email address may contain special characters that are not covered by ASCII. So in the PLAIN method that information is Base64 encoded.

What is actually converted to Base64…

NULL-BYTE + USERNAME + NULL-BYTE + PASSWORD

So for John’s case you can easily create the Base64 string using:

printf '\0john@example.org\0summersun' | base64

As a result you will get the exact same string you used above with “AUTH PLAIN”.

The submission port

Although I have been talking about SMTP on port 25 to relay mails, it is actually not the proper way. End users should not use port 25 but rather the submission service on TCP port 587 (as described in RFC 4409). The idea is to use port 25 for transporting email (MTA = mail transport agent) from server to server and port 587 for submitting (MSA = mail submission agent) email from a user to a mail server.

For comparison:

TCP Port	25	587
Service name	SMTP	Submission
Encryption	Optional	Mandatory
Authentication	Optional	Mandatory
Meant for	Server-to-Server	User-to-Server
Your ISP	may block this port	should allows this port

I hope that makes the distinction a bit clearer. So let’s enable the submission service. All Postfix services are declared in the /etc/postfix/master.cf file. Please edit the file and look for the submission section that is commented out by default. Turn that section into the following. Basically I removed the ‘#’ character on all lines of this section and removed the lines with the mua_* variables.

submission inet n       -       y       -       -       smtpd
  -o syslog_name=postfix/submission
  -o smtpd_tls_security_level=encrypt
  -o smtpd_sasl_auth_enable=yes
  -o smtpd_tls_auth_only=yes
  -o smtpd_reject_unlisted_recipient=no
  -o smtpd_client_restrictions=
  -o smtpd_helo_restrictions=
  -o smtpd_sender_restrictions=
  -o smtpd_relay_restrictions=
  -o smtpd_recipient_restrictions=permit_sasl_authenticated,reject
  -o milter_macro_daemon_name=ORIGINATING

Make sure to start the first line in the first column and indent the following lines.

This service uses the “smtpd” daemon (see the last word of the first line) which is the piece of software that responds if you open an SMTP connection on TCP port 25. But it gets a few extra options set…

• in the /var/log/mail.log log file you will see the connections to the submission port as “postfix/submission” (the syslog_name setting above)
• enforce encryption on this port (smtpd_tls_security_level)
• enable authentication (smtpd_sasl_auth_enable)
• enforce encryption during authentication (smtpd_tls_auth_only)
• allow sending emails to recipients outside of this mail server (smtpd_reject_unlisted_recipient)
• remove special restrictions (smtpd_*_restrictions)
• allow relaying if the sender was authenticated (smtpd_recipient_restrictions)
• send the string ORIGINATING to milter services (milter_macro_daemon_name) – you can just leave it like that

Restart the Postfix server:

systemctl restart postfix

Your users can now use the submission port to send email. They just use the port 587 in their mail clients instead of port 25.

Feel free to send a test mail using the submission port and encryption. You will need to install the libnet-ssleay-perl package first, so that SWAKS can speak TLS:

apt install -y libnet-ssleay-perl

swaks --server localhost --to john@example.org --port 587 -tls --auth-user john@example.org --auth-password summersun

Protecting against forged sender addresses

A “forged” sender address means that someone claims to be someone else. Let’s say that John has authenticated and the mail server trusts him. Nothing keeps John from impersonating someone else and sending email in his name? Most email service providers have restrictions that you can only send emails if the sender matches your actual email address. Let’s do that, too.

Postfix provides a setting called smtpd_sender_login_maps for that purpose. From the “maps” part you can deduce that it expects a mapping again. This time the two columns mean…

• Left column: who you claim to be (email’s sender address)
• Right column: who you logged in as (user name after authentiation)

As we use the email address also as a user name we simply need a mapping where the email address is listed on both sides. Fortunately we already have a mapping like that: our email2email mapping that we used earlier as a workaround for catchall forwardings. Let’s reuse it. Please run…

postconf smtpd_sender_login_maps=mysql:/etc/postfix/mysql-email2email.cf

This sets the parameter both for the SMTP port (25) and the submission port (587). Defining these maps is not enough though. You also need to make Postfix act on this. Edit the /etc/postfix/master.cf again and in the submission section add the following option. Make sure the line is indented like all other options:

 -o smtpd_sender_restrictions=reject_sender_login_mismatch,permit_sasl_authenticated,reject

Restart Postfix after this change:

systemctl restart postfix

You can now try to send email as a different user than you are logged in. Let’s us swaks again to send a spoofed email:

swaks --server localhost --from john@example.com --to john@example.org --port 587 -tls --auth-user john@example.org --auth-password summersun

Please note the bold-printed part above. John’s actual email address is not john@example.com but john@example.org. So it must be rejected. The server should tell you:

~> MAIL FROM:john@example.com
<~ 250 2.1.0 Ok
~> RCPT TO:john@example.org
<~* 553 5.7.1 john@example.com: Sender address rejected: not owned by user john@example.org

Sorry if the mixture of example.org and example.com in my examples makes the guide less readable. But I can’t use any real world domains here. Please just note that sometimes it’s “.com” and sometimes “.org”.

Of course you can test your new security feature in the Roundcube web mail interface as well. In Settings/Identities you can create another sender email address as in:

With this wrong sender address any email should get rejected. Your mail.log would read something like:

NOQUEUE: reject: RCPT from foo.bar[…]: 553 5.7.1 <someoneelse@example.com>: Sender address rejected: not owned by user john@example.org; from=<someoneelse@example.com> to=<…>