If you are a developer working on a marketing or growth team, you have probably implemented this rule at least once: Only accept business
If you are a developer working on a marketing or growth team, you have probably implemented this rule at least once: Only accept business email addresses.
It usually comes from a reasonable request. Marketing wants better lead quality. Sales is tired of fake demo requests. Operations wants cleaner CRM data. Blocking Gmail and other free providers feels like a simple way to raise the bar.
The problem is that email domain restrictions do not actually protect your forms. They optimize for perception, not for abuse resistance. And as the person responsible for the implementation, you are the one left holding the risk.
At Arcjet, we work with teams who are surprised to learn that their marketing forms are often one of the most exposed parts of their application surface. Requiring a work email does not change that reality.
Email domain filtering checks the text after the @ symbol in an email address. If it matches a denylist such as gmail.com, the submission is rejected. From a marketing perspective, that can reduce obvious noise, can slightly improve reported lead quality metrics, and can make stakeholders feel like the funnel is more controlled.
This may seem like a simple string comparison, but did you know that Arcjet tracks over 3,000 free email domains and almost 100,000 disposable email domains? Did you know those lists change every few hours? Arcjet’s systems made changes to those lists with hundreds of domains added based on new domains registered and removed based on delivery reports every day?
Checking for Gmail and Yahoo doesn’t cut it.
Tracking domains is one thing, but automated bots do not think in terms of email providers. They think in terms of endpoints.
If your form posts to an API route, that route can be targeted directly. Automation scripts can skip your frontend entirely and submit requests straight to your backend.
Registering disposable domains takes minutes. Scripts can generate thousands of realistic looking company email addresses. Most spam campaigns do not need working inboxes. They just need your server to accept the request.
If your protection strategy depends on a static list of blocked domains, you are solving yesterday’s problem while exposing today’s endpoint. As a developer, that means the real risk is not fake looking emails. The real risk is unprotected request handling.
Blocking free email domains does not just affect malicious traffic. It affects real users.
Developers often evaluate tools with personal email addresses before involving their company, early stage founders may not yet have a formal domain, and consultants and contractors frequently use personal accounts for testing.
If you are building for a developer audience, some of your highest intent users may start with Gmail. When you block them, marketing may see fewer low quality leads, but you may also see fewer high potential ones. And if abuse still gets through, engineering still has to deal with it. You may end up owning the downside without actually reducing the threat.
Email domain restrictions treat abuse as a formatting issue, but security is rarely about formatting. It is about behavior.
The more useful questions look like this:
Those signals live at the application layer. They require server side enforcement and contextual evaluation. They cannot be answered by checking the email string alone.
If your form writes to a database, triggers a workflow, or syncs to a CRM, it is part of your application surface. It deserves the same protections as any other API endpoint.
If you are the developer responsible for marketing infrastructure, the goal is not to block Gmail, but to protect the endpoint without hurting legitimate conversions. That means starting with server side validation so no submission is trusted based only on frontend checks. It means adding rate limiting to prevent high volume abuse. It means detecting automation based on behavior rather than static lists.
Email domain rules can still be useful as a qualification signal. For example, you might route business domains differently in your CRM, but they should not be your primary line of defense.
At Arcjet, we approach form protection the same way we approach any other application endpoint: by evaluating risk in context. Enforce protections where the request lands to keep friction low for real users while making abuse expensive for attackers.
Arcjet is designed to protect application endpoints, including marketing form routes, directly in your codebase. Instead of relying on fragile deny lists, Arcjet evaluates requests in context. You can apply rate limiting to your demo request endpoint, enforce bot detection on your signup route, and define policies that run server side where the request actually lands.
Because it integrates at the application layer, protection happens alongside your existing route handlers, there is no need to bolt on complex infrastructure or maintain evolving blocklists. Arcjet refreshes its domain reputation database multiple times per day from a variety of sources to keep up with abuse patterns. You define clear rules for how your form endpoints should behave, and Arcjet enforces them consistently.
This means you can keep friction low for legitimate users, including developers testing from personal email accounts, while making automated abuse significantly harder. Email domains can still be used as a signal within your business logic, but with Arcjet they become one factor in a broader risk decision rather than your only filter.
Marketing forms connect directly to revenue systems, feed sales pipelines, trigger onboarding workflows, and shape analytics and forecasting. Remember, they are not cosmetic components, they are entry points into your system.
Blocking free email providers might make a dashboard look cleaner, but it does not make your system safer. If you want real protection, treat your forms like the application endpoints they are. Evaluate behavior. Enforce server side controls. Use email domains as one signal among many, not as a stand in for security.
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