Is your email’s guest list out of date? Why a small setting has a big impact

Imagine your business domain (yourcompany.com) is an exclusive club, and your emails are VIP guests you’re sending out to your customers. To make sure only your real emails get past the bouncers (the spam filters at Google, Microsoft, etc.), you give them a guest list. This list tells them which messengers are allowed to deliver messages from you.

In the world of email, this guest list is called a Sender Policy Framework (SPF) record. It’s a simple but crucial security setting that publicly lists all the services authorized to send email on your behalf—like Google Workspace for your company email, Mailchimp for your newsletters, or Salesforce for your customer updates.

When an email arrives claiming to be from you, the receiving server quickly checks your SPF guest list. If the sender is on the list, the email is welcomed into the inbox. If not, it’s treated as a potential party crasher—and either gets thrown into the spam folder or blocked entirely.

This might sound like a minor technical detail, but keeping this “guest list” clean and current is one of the most important things you can do for your email. Neglecting it can lead to two major problems: your important messages won’t get delivered, and scammers can start impersonating your brand.

Problem #1: Your emails end up in the spam folder

When your SPF record is broken or out of date, even your most important emails can fail the bouncer’s check. This has a real impact on your business:

• Lost sales: That critical proposal or quote you sent? It might be sitting in a client’s junk folder, unnoticed.
• Customer frustration: Password resets, shipping confirmations, and support ticket updates fail to arrive, leading to confused and unhappy customers.
• Damaged reputation: If customers and partners can’t reliably receive your emails, they start to lose trust in your brand’s professionalism.

A common real-world example is the story of Inbound Design Partners. They discovered that legitimate emails from their business Gmail accounts were landing in their clients’ spam folders, putting relationships at risk. The problem? Their email setup was incomplete. One of their domains, which was still connected to their email system, was missing its SPF record entirely. Once they created a proper SPF list for all their domains, their deliverability score shot up, and their emails started reaching the inbox again.

Problem #2: Scammers can easily impersonate you

An even bigger danger of a neglected SPF record is that it leaves the door wide open for cybercriminals. Without a clear guest list, attackers can easily send fake emails that look like they came directly from your company—a technique called “email spoofing”.

This is the starting point for some of the most devastating cyberattacks:

• Phishing scams: Attackers impersonate your brand to trick your customers into clicking malicious links or giving up their passwords. If your domain is constantly used for phishing, your brand’s reputation suffers immense damage.
• Fake invoices: Criminals send fraudulent invoices to your clients that look like they came from your finance department.
• Business email compromise (BEC): This is a highly targeted attack where scammers impersonate a CEO or CFO and trick an employee into wiring large sums of money to a fraudulent account. These attacks have cost companies like Google, Facebook, and Ubiquiti Networks over $100 million.

Attackers actively search for domains with missing or broken SPF records because they are easy targets. Having a properly configured SPF record is your first and most basic line of defense.

How your “guest list” gets messy: Common SPF mistakes

Your SPF record is just a single line of text in your domain’s settings, but a few common mistakes can break it and cause all the problems mentioned above.

• Having more than one guest list: A domain can only have one SPF record. When a new email service is added, some administrators mistakenly create a new record instead of merging it into the existing one. This is like handing the bouncer two conflicting guest lists. He gets confused, gives up, and your email authentication fails.
• Forgetting to remove old services: When you stop using a marketing platform or any other email service, you must remove it from your SPF record. Leaving an old, unused service on your list is a security risk. If that service’s domain expires, an attacker could buy it, and they would then be on your official guest list, allowing them to send authenticated fake emails on your behalf. This exact vulnerability was used in a massive 2024 phishing campaign called “SubdoMailing,” which exploited forgotten entries in the SPF records of major brands like eBay and McAfee.
• Making the list too long: The system that checks your SPF record will only perform up to 10 lookups. Think of it as the bouncer only having the patience to check 10 different clipboards. Every time you use an include statement to add a service like Google or Microsoft, it uses one of those lookups. If your record gets bloated with too many services, it can exceed this limit. When that happens, the check fails, and any services listed after the 10th lookup won’t be authenticated.
• Forgetting about the side doors (subdomains): An SPF record for your main domain (e.g., yourcompany.com) does not automatically apply to your subdomains (e.g., marketing.yourcompany.com or support.yourcompany.com). Each subdomain that sends email needs its own, separate SPF record. Ignoring this leaves your subdomains unprotected and can cause their emails to fail delivery.

A wrinkle in the rules: What if the subdomain’s list is perfect but the main list is broken?

You might think that as long as your marketing subdomain (marketing.yourcompany.com) has a perfect SPF guest list, it doesn’t matter if the main list for yourcompany.com is a mess. Technically, you’re partly right—the bouncer for your subdomain’s emails will check the subdomain’s list. But in reality, a broken guest list at the main entrance can still cause problems for your VIPs.

Here’s why:

• Reputation is holistic: Email providers like Gmail and Outlook don’t just look at one list; they judge the reputation of your entire “club.” If the main entrance has a broken, error-filled guest list, it signals that your club’s management is sloppy. This poor reputation can splash onto your subdomains, making their emails look more suspicious, even if their own lists are perfect.
• The master security policy (DMARC): Your main security policy (known as DMARC) is usually set at the root domain and is inherited by all its subdomains. If an email from your subdomain has an authentication issue, the receiving server looks to this master policy. A broken SPF record on the root domain can cause the entire security check to fail, even for the subdomain’s email.

It’s like having a pristine guest list for the VIP room while the front door of the club has fallen off its hinges. It undermines the security and trust of the entire operation. The takeaway is simple: every guest list for every door—main entrance and side doors alike—needs to be clean and correct.

The hidden danger: When your own subdomain is used against you

This brings us to a more advanced danger: what happens when an attacker doesn’t just pretend to be you, but actually gets control of a forgotten part of your domain?

Imagine you have an old, unused subdomain, like promo2022.yourcompany.com. If it’s not properly secured, an attacker can hijack it. But here’s the truly malicious part: they don’t just send spam from it. They go into its settings and edit its SPF guest list, adding a whole list of other known spammer domains.

Now, all those spammers are “authorized” to send email on behalf of your hijacked subdomain. When they send millions of spam messages, recipients complain. Email providers like Gmail see a massive number of spam complaints coming from servers that are officially part of your brand’s “zone.”

This is where the real damage happens. The terrible reputation of that one hijacked subdomain starts to poison the reputation of your entire brand. Even though your main domain (yourcompany.com) is secure, providers see the flood of complaints from your “zone” and start treating all of your emails with suspicion. Suddenly, your important, legitimate messages start landing in spam folders.

This concept of an “SPF Zone” is a useful way to think about it. It’s not just about your main domain; it’s about the entire collection of domains and subdomains you control. A vulnerability in one forgotten corner can bring down the reputation of the whole operation.

How to keep your SPF record clean and effective

Maintaining your SPF record isn’t a one-time task; it’s a simple but ongoing process. Here’s a straightforward plan:

1. Know who sends email for you. Make a complete list of every service you use that sends email on your behalf. This includes your main email provider (Google Workspace, Microsoft 365), marketing platforms, CRMs, customer support tools, and even your website’s contact form.
2. Create your master guest list. Combine all your authorized senders into a single SPF record. If you already have a record, simply add new services to it. Never create a second one. For example, if you use Google and SendGrid, your record would include both.
3. Publish your record. Log in to your domain provider (like GoDaddy, Cloudflare, or Namecheap) and find the DNS settings. There, you’ll add or edit the TXT record for your domain and paste in your single, consolidated SPF string.
4. Check your work. After you’ve published the record, use a free online tool like MxToolbox or PowerDMARC’s SPF Checker to make sure it’s set up correctly and doesn’t have any errors.
5. Make it a habit. Review your SPF record at least once a quarter. Make it a standard part of your process: whenever you sign up for a new service that sends email, add it to the record. When you stop using one, remove it immediately.

Beyond the guest list: Building a full security team

SPF is your foundational security guard, but for complete protection, it works as part of a team with two other email authentication standards: DKIM and DMARC.

• DKIM is the tamper-proof seal: DKIM adds a unique, unforgeable digital signature to your emails. This proves that the message is genuinely from you and that its contents haven’t been altered along the way.
• DMARC is the head of security: DMARC is the final policy that tells receiving servers what to do. It checks that an email passes both the guest list check (SPF) and the seal check (DKIM). Then, it enforces your rules: either reject fake emails outright or quarantine them in the spam folder. DMARC also sends you reports, giving you valuable insight into who is sending email from your domain so you can spot problems early.

The bottom line

Your SPF record may be a small piece of your digital presence, but its impact is enormous. A well-maintained record ensures your important communications reach their destination, protects your customers from dangerous phishing scams, and defends your brand’s hard-earned reputation. Take a few minutes today to check your “guest list”—it’s one of the simplest and most effective steps you can take to secure your email.