Using an SMTP cloaking service to protect your mail server

Simple Mail Transport Protocol (SMTP) is the underlying technology that acts as the foundation for email delivery. In simple terms, it is the language mail servers use to communicate with each other and to move email messages across the Internet.

According to Wikipedia, 78% of all emails sent are spam. As a consequence, mail servers must constantly evolve to try to stay ahead.

If you operate your own mail server, there are several potential risks you may face:

• Excessive email delivery attempts by spammers or other illegitimate users could consume considerable bandwidth and processing power on your mail server.
• Any security issues with your mail server software (such as buffer overflows or injection vulnerabilities) could compromise the integrity of your entire mail system.
• If your server is hosted on a cable or DSL broadband connection, there is a possibility that your ISP will block port 25 (the SMTP port) in an attempt to prevent spammers from using your network. This would render your server unusable as it would no longer be able to receive mail.

An SMTP cloaking service works by placing one or more public SMTP servers in front of your own mail server. Public servers receive all mail destined for your domain, perform the necessary antispam and antivirus checks, and only pass the mail to your own mail server if the message appears to be good.

This arrangement offers some considerable advantages:

• The outside world does not have direct access to your mail server; they don’t even know what kind of mail server software you are running.
• Its bandwidth is reduced, since only those messages that have passed the tests defined by public servers will be delivered to their own server.
• You can run your mail server on a port other than 25, perfect if your ISP is one of those that blocks connections to port 25.
• A good SMTP cloaking service will provide multiple public SMTP servers, providing you with additional redundancy in case one of your servers is down.
• If your own server is down for any reason, mail will be held by public SMTP servers and automatically delivered to your own server when it is back online, providing you with a mail backup system and ensuring you never miss any email as a result of problems with your own mail server.

There is a potentially significant drawback to using an SMTP cloaking service:

If the cloaking service does not have a list of all valid users on your domain, it will need to forward every email it receives to its server, regardless of whether the recipient exists or not. Some SMTP cloaking services will allow you to provide a valid list of mailboxes in your domain; this is certainly a worthwhile exercise, as it allows public servants to reject any mail sent to non-existent recipients. However, you will need to think about how you will update this list each time you add a new user to your mail server; this is probably best discussed with your SMTP cloaking service provider, as the procedure will be different for each provider.

Choosing an SMTP cloaking service provider

• You should expect to pay between £ 40- £ 60 ($ 55- $ 85 USD) per year per domain, depending on the features and support provided. Some companies may charge more for premium levels of support or additional features.
• Beware of limits on the amount of email traffic and whether you will incur additional charges for excessive email volumes or bandwidth usage.
• Does the service allow you to specify a list of valid mailboxes on your domain? And if so, how often can it be updated?
• Does the service include any anti-spam or antivirus measures? While anti-spam solutions can help reduce spam, those that are overly sensitive could unknowingly reject legitimate emails.

If you have any questions not covered in this article, my best advice is to reach out to various SMTP cloaking service providers to discuss your own specific requirements. Everyone’s needs are different and the potential benefits you could gain from having an SMTP cloaking service will depend on your own individual circumstances.