Email Security: Common Issues and Best Practices

Email Security: Common Issues and Best Practices
Valeriia Dziubenko Valeriia Dziubenko 18 october 2023, 15:12 342
For experts

Security is paramount when sending emails. With billions of active users worldwide, email is the primary target of a majority of cyber attacks. Hence, you must take extra steps to make your SMTP server as secure as can be and protect your correspondence from prying eyes.

This article will explain what you need to know about email security. We’ll discuss the common cyber threats and outline the steps to secure your correspondence as much as possible.

Common Threats to Email Security

The list of cyber threats is expansive. Your system needs protection from common threats like malware, DDoS attacks, phishing, spam, etc. We’ll dive deeper into these threats below.


Malware is a short form of “malicious software.” Hackers look for vulnerable servers to infect with malware for nefarious intent. It could be a virus that disrupts your server and stops messages from flowing freely, or a program designed to steal sensitive information from your server and sell it. Hackers could also try to take control of your SMTP server to send more malicious messages to people.

Regardless of the type of malware, you must protect your system from being infected.

DDoS Attack

DDoS is an abbreviation for “Distributed Denial of Service.” It’s when malicious actors flood a server with too many requests that it can't handle, causing it to stop functioning.

Hackers could target your email server with a DDoS attack; if successful, your server won’t be able to send and receive messages.

DDoS is among the most common attacks on email servers or any other server type. Hence, relevant protection for your SMTP server is necessary.


Phishing is when malicious actors impersonate a legitimate brand to trick email users into giving away sensitive information or installing malware on their devices. An unsuspecting customer may receive a normally looking message requesting a certain action in their bank account, but the link will take them to a visually identical spoofing site. This type of attack may be especially hard to detect.


Spam is any unwanted message sent to email addresses, usually in bulk. It is closely related to phishing and malware, as spam messages often contain malicious attachments or phishing links.

To mitigate the threat, network administrators must install anti-spam filters and use reputable blacklists to keep spammers away.


Ransomware is a rapidly growing type of malware attack. Hackers gain unauthorized access to a computer, encrypt the data, and demand a ransom from the owner to unlock the it. Big businesses are often the targets because they can afford to pay sizable ransoms.

Ransomware actors are becoming increasingly sophisticated, so your IT infrastructure needs as much protection as possible.

SMTP Injection

SMTP Injection is when hackers inject malicious commands into vulnerable SMTP servers or other software using flaws in various SMTP implementations. The commands are used to penetrate the system and send spam or phishing emails to unsuspecting users.

Ensure your systems are updated with the latest security patches. This protection is mandatory to prevent malicious actors from sneakingly using your servers to send malicious messages and tank your reputation.

SMTP Traffic Interception

If your SMTP traffic is not encrypted, a hacker may intercept the data in transmission to extract any sensitive information contained within. A so-called “Man In The Middle” (MITM) attack is also possible, when a malicious actor alters the data on the fly for nefarious purposes.

This type of attack may happen even if you enable encrypted communication, but the keys you’re using are compromised or the cryptographic protocol is flawed.

Ways To Make Your SMTP Server Secure

We have discussed the most common threats to protect your server from; these threats can cause significant damage to your business. We’ve repeatedly stated that proper protection is vital for your SMTP server and other software. Now, it’s time to learn about the ways to do that.

There’s no single method of securing an email server. Instead, you need to take diverse measures to protect your communications on all possible ends.

The most important ways to protect your SMTP server are outlined below.

Encrypt Your SMTP Traffic

SMTP lacks inherent security features by default, making your emails vulnerable to anyone capable of intercepting the network traffic. But you can use SMTPS to send messages securely; this variant of SMTP adds an extra layer of Transport Layer Security (TLS) encryption to your connection. This way, all data will be encrypted, and whoever intercepts it will see only gibberish.

Set Up SMTP Authentication

You can set up email authentication for your servers. These techniques provide a way to verify that an email you receive really comes from a particular domain.

The SMTP authentication protocols currently in use are:

  • Sender Policy Framework (SPF): Using DNS TXT records to specify the IP addresses approved to send messages from a particular domain.
  • Domain Keys Identified Mail (DKIM): Using cryptography to digitally sign messages helps verify the integrity of its headers.
  • Domain-based Message Authentication, Reporting And Conformance (DMARC): Setting policies for accepting or rejecting messages that fail SPF or DKIM check.

Configure mail relay options carefully

SMTP servers come with open relay by default, but you should configure your email relay options for better control of your SMTP server. Configuring your relay options lets you choose which domain names your mail server can send messages to. It also ensures that only authorized users can relay emails through your SMTP server.

Be careful when configuring your mail relay options. Ensure you choose the correct configurations and double-check before deploying them. Any misconfigurations can make your server vulnerable to abuse by malicious actors.

Activate Reverse DNS

Most email servers use DNS lookups to verify incoming emails. Upon receiving a message, the recipient's SMTP server checks to see if the sending IP address matches the source domain name.

There are two main types of DNS lookups: forward and reverse. A forward DNS lookup takes a domain name and matches it to an IP address. A reverse DNS lookup does the opposite: it takes an IP address and matches it to a domain name.

You can set up a reverse DNS record to match your IP address to your domain name. This record makes it easy for email servers to verify your identity once a message from your server comes knocking at their door.

Limit Connections to Your SMTP Server

You can limit the number of simultaneous connections from a single IP address to protect your email server from threats. This way, even if a malicious actor gains access to your system, they won’t be able to use it to send messages in large volume.

Let’s say your SMTP server allows up to 1,000 emails per hour using a single connection. If you limit your connections to just 2 per account, a malicious actor won’t be able to send more than 2,000 messages hourly. Without this limit, they can send tens of thousands of emails from your server and damage your sending reputation.

Maintain Local IP Blocklists

You can create blocklists of IP addresses for your mail server. The IP addresses on a blocklist are those you have identified as a source of spam and malware. Your SMTP server will refuse to communicate with any IP address on this blocklist, preventing the malicious actors from sending malware to your users.

Hackers constantly change their IP addresses, so you must keep updating your list to include any new malicious address you detect. Of course, you can’t block all of those, but you can thwart as many as possible, especially the ones that have attempted to abuse your server.

Encrypt POP3 and IMAP for privacy

SMTP is one of the three main email protocols. The other two are Post Office Protocol ver. 3 (POP3) and Internet Message Access Protocol (IMAP). SMTP transmits emails between servers, while POP3 and IMAP retrieve incoming messages from your server to your email client.

POP3 and IMAP are also insecure by design. But you can add an extra layer of security via TLS/SSL for protection. This way, hackers won’t be able to intercept sensitive information coming into your email server.

Backup Your Files

You should create regular backups of your mail server, including its contents and configurations. This way, if your server is somehow breached, you can restore it to the previously backed-up version and minimize data loss.

For instance, if you fall victim to ransomware but have a recent backup, you can ignore the hacker and simply restore the backup.

Install Antivirus Software

You should also install antivirus software on both your mail servers and PCs to protect them from cyber threats. The software will check every inbound and outbound email for malware and tackles anyone it detects. It also provides filters to identify messages likely to be spam and send them to the spam folders where they belong.

Examples of antivirus software vendors include Symantec, Sophos, Trend Micro, etc.

Earlier, we mentioned TLS/SSL as an encryption protocol for your communications. However, with TLS/SSL encryption the message is only encrypted during transit from your server to the recipient's. After arriving at the recipient's server, it becomes unencrypted and thus may be read by anyone with access to the server’s hard drive.

But end-to-end encryption guarantees all-around protection. An email will be encrypted at every stage, from the moment it’s sent and up until it’s opened by the addressee. If any third party intercepts the data, it’ll be useless because they don’t have the keys to decrypt the message. Let’s see how end-to-end encryption works for a hypothetical email user John, who is sending a congratulatory email to Jane.

  • Jane (the recipient) generates unique public and private keys. She’ll keep the private key in a secure place but share the public key with John (or anyone she wants to communicate with her).
  • John gets Jane’s public key and uses it to encrypt the email he sends to her. Encryption converts all the email content into ciphertext as it’s transmitted from John’s email server to Jane’s. If hackers intercept the contents, they’ll see a random set of characters that don’t make sense. And as you might guess, the public key is not suitable for decoding the message.
  • The email arrives at Jane’s mailbox. She uses her private key to decrypt the message, converting it from ciphertext into original form.

The inverse process occurs if Jane replies to the email John sent her. In that case, Jane would use John’s public key to encrypt the message, and John would use his private key to decrypt and read it.

Let’s now discuss the most popular end-to-end encryption methods.


S/MIME is an abbreviation for Secure/Multipurpose Internet Mail Extensions. It was originally proposed in 1995 and is now supported by most popular email clients, with the exception of web-based email apps. S/MIME encrypts all the contents of an email, making it impossible for hackers to decrypt unless they get the encryption keys.

This technique also allows you to add a digital signature to your email to help the recipient verify that it comes from you and has not been altered after sending.


PGP stands for Pretty Good Privacy. It’s a cryptographic utility developed by Philip Zimmerman in 1991

PGP uses public and private keys, just like the S/MIME protocol. The difference is that PGP only secures text, while S/MIME secures both text and any attachments like pictures, videos, audio, etc.

PGP is less effective than S/MIME due to its limited capabilities, but it’s often easier to implement.


Bitmessage is a decentralized encrypted communications protocol. This protocol was developed in 2012, making it one of the newer end-to-end encryption technologies. It was conceived by Jonathan Warren, a software developer, who was inspired by the design of Bitcoin, a decentralized cryptocurrency.

Bitmessage combines asymmetric cryptography with blockchain technology added on top. It is not an email client per se, but can be used as a secure alternative for both one-to-one and one-to-many communication.


We have outlined the basics of email security. We discussed the common security threats you should be aware of and the tips to protect your email communications as much as possible. We also explained how end-to-end encryption guarantees the highest level of security and gave a few examples of end-to-end encryption techniques.

Above all, it’s essential to choose a secure SMTP service. UniOne is one such service with a good track record regarding security. Our platform incorporates advanced security protocols to protect your data and ensure you send emails safely and reliably.

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