Every SaaS product with a registration form eventually faces the same problem. Bots sign up using real people's email addresses. Your application dutifully sends a verification email to an address that belongs to a person who never asked for it. That person marks it as spam. Your sender reputation takes the hit.
This is not a hypothetical. The connection between bot abuse at registration and damage to email deliverability is direct, measurable, and increasingly difficult to recover from. If you are building a product that sends transactional email, bot deterrence is not just a security concern. It's a concern about your reputation.
The Bot Traffic Reality
The scale of automated traffic on the internet has crossed a threshold that changes how every web application needs to think about its public-facing forms.
According to the Imperva 2025 Bad Bot Report, automated traffic surpassed human activity for the first time in a decade, accounting for 51% of all web traffic. Bad bots specifically now represent 37% of all internet traffic, up from just over 30% in 2023. That is a six-year streak of year-over-year growth in malicious automation.
The rise of AI and large language models has accelerated this trend. LLMs have made it cheaper and easier to create bots that mimic human behavior, fill out forms with realistic-looking data, and rotate through residential proxies to avoid detection. Cloudflare's 2025 Year in Review reported that AI-driven crawling activity increased by over 15x during the year, with 40% of global bot traffic originating from the United States alone.
Registration forms are a primary target. Bots probe them to test stolen credentials, abuse free trials, scrape application logic behind authentication walls, or simply use your email infrastructure as a relay. The common thread across all of these attacks is that they trigger your system to send email to addresses that did not ask for it.
How Bot Signups Damage Email Deliverability
Here is the chain of events that turns a bot registration into a deliverability crisis.
A bot submits your registration form using a real person's email address. This is not a throwaway or disposable address. It belongs to someone who has no idea your product exists. Your application sends a verification email to that address, exactly as designed. The recipient sees an unexpected email from a service they never signed up for. They do what any reasonable person would do: they hit "Report Spam."
One complaint is not a problem. But bots do not submit one form. They submit hundreds or thousands, often in bursts, using harvested email lists. Each submission triggers an outbound email. Each email that gets reported as spam erodes your sender reputation with the mailbox providers that matter most.
The thresholds are tighter than most developers realize. In February 2024, Google and Yahoo began enforcing new requirements for bulk senders. Google now enforces a hard spam complaint ceiling of 0.3%, with a recommended target below 0.1%. Microsoft followed with similar rules for Outlook, Hotmail, and Live.com addresses in May 2025. As of November 2025, Gmail permanently rejects non-compliant emails rather than just delaying them.
The math is unforgiving. If you send 10,000 emails and just 30 recipients mark your message as spam, you have hit the 0.3% threshold. For a SaaS product in its growth phase, a single bot attack that generates a few hundred fake signups over a weekend can push you past that line.
Once your sender reputation is damaged, the consequences extend far beyond the spam folder. Your password reset emails stop arriving. Your onboarding sequences get filtered. Your invoice notifications disappear. The bot never needed to complete email verification or access your application. The damage was done the moment your system sent the email.
Email Verification Is Not Enough
A common response to bot registration abuse is to require email verification before activating an account. This is a good practice, but it does not solve the deliverability problem because the damage happens before verification.
The verification email itself is the weapon. It does not matter that the bot never clicks the confirmation link. What matters is that your domain sent an unsolicited email to a real person, and that person told their mailbox provider it was spam. Email verification prevents unauthorized access to your application. It does not prevent the reputational harm of sending unwanted email in the first place.
This is the core insight that many teams miss. The goal is not just to keep bots out of your application. The goal is to prevent your application from sending email to addresses that were submitted by bots. Those are two different problems, and they require different layers of defense.
A Layered Approach to Registration Defense
No single technique stops all bot traffic. Effective protection requires multiple layers that work together to reduce the volume of illegitimate submissions before your system ever sends an email.
Enable bot protection at the hosting or firewall level. This is the lowest-effort, highest-impact first step. Most hosting providers and CDNs (Cloudflare, Vercel, AWS WAF, Netlify) offer a one-click bot denial or bot fight mode that blocks known automated traffic before it reaches your application. If your infrastructure supports it, turn it on.
- One caveat: blanket bot denial also blocks legitimate crawlers, including those from OpenAI, Anthropic, Google Gemini, and other AI services that index your content for AI-powered search results.
If it matters that the marketing-facing pages of your site remain crawlable for AI search optimization, consider putting your application behind a subdomain. For example, your marketing site lives at cuttlesoft.com a place where crawlers are welcome, while your registration flow and authenticated application live at app.cuttlesoft.com with aggressive bot protection enabled. This gives you the best of both worlds: AI discoverability for your public content and strong bot denial on the endpoints that matter.
Rate limiting on your registration endpoint. This is the most basic defense and the one most often missing. Cap the number of registration attempts from a single IP address or session within a given time window. This will not stop distributed attacks, but it raises the cost for unsophisticated bots and limits the blast radius of any single source.
Challenge mechanisms. CAPTCHA has been the default for years, but traditional reCAPTCHA is increasingly bypassed by AI-powered bots and low-cost CAPTCHA-solving farms. Cloudflare Turnstile and similar invisible challenge tools offer better protection with less friction for real users. They analyze browser behavior and environmental signals rather than asking users to identify traffic lights.
Honeypot fields. Add a hidden form field that is invisible to human users but gets filled in by bots that programmatically complete every field on the page. Any submission with data in the honeypot field gets silently rejected before an email is sent. This is easy to implement, free, and has zero impact on user experience. It will not stop targeted attacks, but it catches a surprising volume of automated submissions.
Email validation before sending. Before your application sends a verification email, validate the submitted address. Check for disposable email domains (there are maintained blocklists of thousands of temporary email providers), verify that the domain has valid MX records, and reject addresses with obviously invalid syntax. This prevents your system from sending email to addresses that are guaranteed to bounce or belong to throwaway services. Services like ZeroBounce, Kickbox, and Mailgun's email verification API can do this in real time at the point of registration.
Behavioral analysis. Bots fill out forms differently than humans. They tend to complete fields instantly, ignore mouse movement patterns, and skip the natural pauses between field entries. Client-side JavaScript can measure time-to-submit, keystroke dynamics, and interaction patterns. Submissions that complete in under a few seconds are almost certainly automated.
Separate your email streams. Use different subdomains for transactional email and marketing email. This isolates your sender reputation so that spam complaints against one stream do not poison the other. For example, we could structure our sending domains like this:
- Verification Emails:
verify.cuttlesoft.com - Account Alerts and Password Resets:
notify.cuttlesoft.com - Newsletters and Marketing:
mail.cuttlesoft.com
If bot-triggered verification emails damage the reputation of verify.cuttlesoft.com, password resets sent from notify.cuttlesoft.com remain unaffected. The key is that each subdomain builds its own reputation with mailbox providers independently. Nearly half of high-volume senders already separate email streams on different subdomains, according to Mailgun's State of Email Deliverability report.
Monitoring: What to Watch
Even with defenses in place, you need visibility into what is happening. Set up Google Postmaster Tools for your sending domain. It surfaces your spam complaint rate, domain reputation, and authentication status as Google sees them. If you are sending to Yahoo addresses, enroll in Yahoo's Complaint Feedback Loop through their Sender Hub to receive reports when recipients flag your email.
On the application side, track registration velocity. A sudden spike in signups with no corresponding marketing activity is a signal. Monitor bounce rates on verification emails. A sharp increase in hard bounces suggests bots are submitting invalid or nonexistent addresses. Set up alerts for both so you can respond before the damage compounds.
The Cost of Doing Nothing
The temptation is to treat bot registration as a future problem, something to address after launch or after the first incident. The reality is that recovering from damage to sender reputation can take weeks or months. Google, Yahoo, and Microsoft do not offer a quick fix. You have to demonstrate sustained good sending behavior over time before your reputation recovers and your emails start reaching inboxes again.
During that recovery period, your real users are not receiving password resets. Your onboarding emails are landing in spam. Your trial-to-paid conversion rate drops because new users never see the emails that guide them through setup. The downstream revenue impact is real and disproportionate to the cost of implementing basic bot defenses before you ship.
Bot deterrence at registration is not a security nice-to-have. It is infrastructure protection for your email channel. Every verification email your system sends to an address that did not ask for it is a small bet against your sender reputation. At the scale bots operate today, those bets add up fast.
If you are building a SaaS product and want to get your registration flow and email infrastructure right from the start, we can help. Learn more about our SaaS development services or get in touch to talk about your project.
