How to Secure Your Email Server Against Attacks?

An unsecured server can be a serious email security problem for your organization and one that can severely damage your reputation with customers.

However, keeping the email server secure, whether it's on-premise or on the cloud, is not easy and there are a lot of things to consider. That is why we've prepared this article to give you some useful tips on how to secure your email server against attacks from hackers and the like.

So let's get started.

Avoid Being Open Relay by Correctly Configuring Mail Relay Options

Two computers communicating

One of the things that spammers like the most is a poorly configured mail relay. What this allows them to do is use your mail server as a gateway to send spam messages to others without repercussions.

To avoid your mail server being Open Relay, be sure to correctly configure which domains and IP addresses it can relay mail for, or in other words, for whom the SMTP can forward mail.

Fight Incoming Email Abuse by Using DNSBL Mail Servers

DNSBL stands for DNS-based blacklist, and this can significantly reduce the amount of unsolicited incoming mail.

This is basically a service that will give you a list of domains and IP addresses that are known for being a source of spam

Be sure to use this configuration and check if either the sender domain or IP are known by a DNSBL server such as Spamhaus.

Maintain Your Own Local IP Blacklist

Speaking of blacklists, you should also maintain your local IP blacklist in order to block those pesky spammers.

You'll encounter five challenges to blacklisting IPs:

  1. The attacker changes their IP address
  2. They are using a botnet
  3. They use IP spoofing
  4. You are getting false positives
  5. Wrong IP detection

This, of course, means that maintaining an IP blacklist is not easy and will take time to implement properly, but it's necessary to prevent unsolicited email connections from messing with your network.

Control User Access with SMTP Server

Control who can access your SMTP server

It's also important to configure the mail server in such a way you prevent unauthorized access. This is done by using SMTP authentication, or Simple Mail Transfer Protocol.

SMTP is an Internet standard communication protocol for email transmission and SMTP authentication will restrict your mail server to only users with the right username and password.

For anyone else, the server will remain locked and thus prevent potential abuse.

Prevent Spoofed IP Addresses by Activating SPF

One of the main ways that spammers will cover their tracks is by using a spoofed IP address. To put a stop to this, be sure to use SPF, or Sender Policy Framework.

By activating SPF on the email server, you ensure that the sending server MX (Mail Exchange) record is first validated before the message is sent.

Encrypt POP3 and IMAP Authentication

A big weakness of POP3 (Post Office Protocol) and IMAP (Internet Message Access Protocol) connections is that neither is created to be secure.

As a result, users' passwords are typically transmitted through the mail server in cleartext, or text that is not meant to be encrypted (vs plaintext which is meant to be encrypted).

What this means is that your mail server can be accessed by hackers and other bad actors, so to prevent this implement strong authentication using TLS or Transport Layer Security.

Verify Message Content with SURBL to Protect Against Phishing Attacks

Use SURBL to protect against phishing attacks

SURBL or Spam URI Real-time Block Lists, are not available on all mail servers, but it's definitely a good idea to activate it if your mail server supports it.

This will check incoming emails for malicious links and that way protect you against email phishing attacks and malware.

Limit Connections to the Mail Server

Be sure to also limit the maximum number of SMTP connections to a necessary minimum. Specifically, you should limit:

  1. The total number of connections
  2. How many simulatenous connections you can make
  3. And the max connection rate

What this will help do is protect your email server against DoS attacks.

Block Fake Senders by Activating Reverse DNS Lookup

One tool that can be very useful in blocking messages that show authentication errors or don't pass the address match test is the Reverse DNS Lookup.

By activating Reverse DNS Lookup you tell the SMTP server to verify if the sender's IP address matches the host and domain names that the SMTP client has submitted in the EHLO/HELO command.

The EHLO/HELo is an ESMTP (Extended Simple Mail Transfer Protocol that the mail server sends to identify itself before connecting to other email servers and start sending mail messages to it.

Use at least 2 MX Records for Failover

Finally, you need to ensure that your mail server software has failover in case the MX record is not available.

Having only one MX record is often not enough for uninterrupted flow of mail to a domain, so you should set at least two (one as primary and one as secondary), so if the primary MX fails for some reason, the secondary can kick in and take over.


And there you have it. 1o tips on how to secure your email server against attacks that will greatly improve your organization's email security.

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How do I make my server secure?

To secure your email server:
Configure mail relay options to avoid the server being Open Relay
2. Use DNSBL (DNS-based black list
3. Maintain a local IP blacklist
4. Use SMTP authentication to control who user access to the server
5. Use SPF to prevent IP spoofing
6. Encrypt both POP3 and IMAP connections
7. Use SURBL to verify message content and avoid spear phishing attacks and malware
8. Limit the maximum number of mail server connections you allow
9. Activate Reverse DNS Lookup to block fraudulent senders
10. Use 2 MX records (at least) for failover