Spam mails are a nuisance! They are often silly and non-threatening mails coming to users’ inbox, and all the users may have to do is to delete it or mark it as spam.

But spam mails can quickly turn into terror when attackers spam server. Too much inbound spamming to your mail server can cause the mail queue to fill up and delay email delivery or even crash it.

For smooth mail delivery, it is vital to prevent spam mails from abusing the mail servers. We configure spam filters, custom rules and anti-spam records in our customers’ servers to stop spam to servers.

At Bobcares, we help businesses block spam and avoid mail server failures as a part of our Outsourced Tech Support. It involves 2 activities:

24/7 server monitoring

Our expert teams monitor critical server metrics such as disk usage and server processes round the clock. When we detect an anomaly, like huge mail queue, we immediately check and investigate for spamming, and prevent a server crash.

Periodic audit and preventive maintenance

But waiting for an issue to happen, and jumping in to rescue is not the best way to keep a server stable. That is why we do regular Preventive Sever Maintenance.

Every couple of weeks, or when we detect an abnormal value in server metrics, we audit the server performance and security. We check the server for security vulnerabilities, performance bottlenecks, hardware health issues, etc. and fix the issues, so that a server failure can be prevented.

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How do attackers spam servers?

From exploiting a software vulnerability to massively spamming servers, attacks can come in any form and at any time. That’s why it is crucial for server owners to stay alert 24/7, to protect their servers.

Attackers can resort to malicious scripts, vulnerable email accounts or even spoof email accounts to send spam to servers. Before implementing ways to block spam, it is important to differentiate between spam and valid mails.

As incorrect filters can lead to loss of valid emails too, we pay extra caution while setting up spam filters and custom rules for blocking server spam for our customers.

There are many symptoms that identifies a mail as spam or not, as listed by CAN–SPAM Act:

1. Mails with false or misleading header information
2. Deceptive subject lines and fraud content in mails
3. Sending advertisements without properly identifying
4. Mails without an option for recipients to opt out 
5. Senders who do not honor opt-out requests promptly

To identify how the spamming is happening and who is doing it, Bobcares engineers examine the email patterns from the mail server logs. This is a vital information based on which we formulate the strategy to stop spam in server.

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How to block spam in your server

Combating spam to ensure mail server stability is crucial for server owners to provide smooth mail delivery to their users. At Bobcares, we thwart spamming in our customers’ mail servers using these methods:

1. Configuring Spam filters

Spam filters are software that are installed and configured along with the mail servers to filter spam. By setting custom filters and rules, we configure them to examine the mails reaching the mail server and block spam to server.

The most commonly installed Spam filters are ‘Spamassassin’ and Mailscanner. We install and configure them alongside antivirus software such as ClamAV in mail servers such as Postfix, Exim, MailEnable, etc.

SpamAssassin to block spam

Once SpamAssassin is setup, users can use the ‘Blacklist’ option to discard certain messages based on email addresses.

Block email address in SpamAssassin

Spam filters need to be trained initially by marking mails as Spam or ‘Not spam’, and monitored regularly to see if they are effectively catching all spam. All mails that are tagged as spam will be sent to the junk folder of the user’s mail account.

But simply installing SpamAssassin will not be sufficient to prevent spamming in your server. The default rule-set of SpamAssassin is configured for generic spam and cannot handle all types of spam that hits your mail server.

At Bobcares, we use extended email logging and email header inspection to identify the spam mails that escape the default rule-set and reach the mailboxes. We then configure custom SpamAssassin rules to block those spam mails.

Our 24/7 security experts monitor the mail queue and logs of the servers regularly to identify new patterns of spam headers that are not caught by the spam filters, and keep on improvising the rule sets based on that.

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2. Looking up anti-spam records

Many spam mails are generated out of email spoofing. Spammers can take valid email addresses and send forged mails with ‘From’ addresses as those IDs, without really authenticating them.

Looking up sender domains for anti-spam records help to validate that the mails are originated from genuine senders and not spammers who spoof the email addresses.

To prevent email spoofing, the commonly configured anti-spam records are:

1. SPF (Sender Policy Framework) – SPF allows a domain owner to add a file or record on the server, that is specific to his domain name. Looking up and validating SPF of sender domains helps to avoid spoofed mails.
2. RDNS (Reverse DNS) – The reverse DNS record is setup for the hostname of the mail server to map to its IP address. If someone is trying to trick the mail server, the reverse DNS lookup will fail and that mail can be discarded.
3. DKIM (Domainkeys Identified Mail) – Domain key is an email authentication system. It is used to verify the domain of the email sender and to make sure that the message is not spam.

RDNS lookup

However, by default, many mail servers do not have SPF checks configured or even if they do, the rule set would be generic. For instance, a rule that uses ‘+all’ will let mails from every host to pass, which is not useful to block spam.

For mail servers to process the lookup of anti-spam records efficiently, we configure custom ACLs (Access Control Lists) for SPF checks and implement filtering to exclude whitelisted domains.

At Bobcares, we tweak the mail servers such as Postfix, Exim, Sendmail, etc. to validate emails with these ACLs and spam checks, thus avoiding spam mails from abusing the servers.

3. Using blacklists in mail server

A blacklist, also known as DNSBL or RBL (DNS Blacklist or Realtime Blacklist), is a spam blocking list. If a mail server IP is listed in the blacklist, most mail servers reject mails from that IP.

SpamCop and SpamHaus are two famous DNS blacklists that contain database of spamming servers. We configure the customer mail servers to lookup these blacklists and to block spam from those blacklisted servers.

Configuring blacklists in ACL

ACLs are used to configure blacklists in mail servers. In addition to Spamhaus and SpamCop, we configure additional blacklists for each customer server based on the source of the spam coming to them.

4. Blocking IPs

In some worst scenarios, where the spamming is out of control and cannot be prevented with spam filters or ACLs alone, we resort to more security measures such as blocking the spamming IPs using firewalls, changing the mail server port, etc.

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Best practices to keep your inbox safe from spam

Alongside blocking spam mails in the server, we also advise the end customers to follow these best practices to block spam from their level.

1. Block sender email address

Every email client have features to block a problematic sender email address. Usually this option to block a sender comes along with the mail itself.

For instance, in Outlook, when you no longer want to see messages from someone, you can block that sender by right-clicking the message and using the option ‘Junk > Block Sender’.

Block sender in Outlook

2. Never reply or click any links in spam mail

Spam mails should be ignored and not replied to. Also, caution should be exercised not to click the links or images in spam mail. Otherwise spammers will confirm that yours is a valid email account.

3. Don’t subscribe to mailing lists

Many commonly visited sites such as online shopping, news sites, blogs, etc. require you to sign up in them, and you may unknowingly end up being in their mailing list or newsletters.

While some newsletters may be relevant, most of them would be advertisements which you don’t prefer to see. Not subscribing to such lists helps to reduce unwanted mails from reaching your inbox.

4. Mark as spam

This is one of the most important feature to block spam. Every efficient mail server would have a Spam filter system in place, and its important that you make use of this feature to avoid spam mails.

Options in the email message, such as ‘Report Spam’, ‘Mark as Junk’, etc. can be used to train the Spam filter to identify that the mail is a spam and prevent it from hitting your inbox after that.

Mark a Spam mail as Junk

5. Caution in disclosing your email address

Care should be exercised regarding where you disclose your email address. Submitting it in almost all websites and locations is like inviting more spam mails to your account.

It is also advisable to use a secondary email address for trivial purposes like newsletter subscriptions or shopping sites and to reserve your primary email account for more priority mails.

6. Avoid default email addresses

Avoid using easy-to-guess email addresses. For instance, email ids such as “mary@mary.com” are easy to attract spammers and can end up receiving too many junk mails.

7. Monitor what others are doing on your behalf

When you authorize marketing or sales agencies for activities such as business promotions, you have to ensure that they don’t misuse your email address or account.

Not exercising adequate caution or control may land you in trouble. You may end up flooded with a lot of spam mails if you overlook such security aspects.

Conclusion

A few proactive and best practices at user end, coupled with a strong and effective Spam filter in mail server, can block spam mails from affecting your customers.

Bobcares helps online businesses of all sizes achieve world-class security and uptime, using tried and tested solutions. If you’d like to know how to make your mail servers secure and efficient, we’d be happy to talk to you.