Every SaaS Eventually Builds Email Validation. Here's Why.

A SaaS founder recently shared a security update for their platform: they'd just added profane email blocking, checks against blacklisted mailing lists, and a rule to block signups when someone uses more than three email aliases. This wasn't a security company - it was a data tool. But like every SaaS that grows past its first few hundred users, they had to start dealing with email abuse.

The pattern every SaaS founder recognizes

It always starts the same way. You launch, you get signups, and everything looks great. Then you start noticing things:

• Someone signs up with an obviously fake or offensive email address. Your welcome email bounces or, worse, you've now got profanity sitting in your database.
• Your free trial numbers look amazing, but the same person is burning through trials with user+1@gmail.com, user+2@gmail.com, user+3@gmail.com.
• A wave of signups comes in from disposable email providers - tempmail.com, guerrillamail.info, throwaway.email - and none of them ever convert.
• Your mailing list costs climb because half your "users" are junk addresses that never open an email.

At this point, every founder does the same thing: they start building email validation into their signup flow. It's never planned. It's always reactive. And it always turns out to be more work than expected.

The DIY approach and its limits

The first instinct is to handle it yourself. Block a few known disposable domains. Add a regex for profanity. Count aliases per base address. It works - for a while.

Here's where it breaks down:

• Disposable email domains multiply constantly. There are over 150,000 known disposable email domains, and new ones appear daily. That list you found on GitHub with 3,000 entries? It's covering maybe 2% of the problem.
• Alias detection is more than plus-addressing. Gmail's dot trick (u.s.e.r@gmail.com = user@gmail.com) is one thing. But some providers support completely different aliasing schemes. And some disposable services generate unique addresses that don't look like aliases at all.
• Profanity filters have edge cases forever. Different languages, creative misspellings, Unicode lookalikes. You'll be updating that regex until the end of time.
• Blacklisted mailing lists change. An email that's clean today might land on a spam trap list next week. Static checks at signup don't catch this.

The founder in this case built all of these checks themselves. That's fine for a solo developer or a small team that's comfortable maintaining security infrastructure. But it's engineering time that isn't going toward the actual product.

What the numbers look like

When a SaaS doesn't validate emails at signup, the costs show up everywhere:

• Email delivery costs: Services like SendGrid, Mailgun, and Postmark charge per email. Sending onboarding sequences to disposable addresses is money in the bin.
• Inflated metrics: Your "10,000 signups this month" number looks great until you realize 30% are throwaway accounts that never logged in twice.
• Trial abuse: A competitor or a determined freeloader can create unlimited trials with disposable emails - a classic multi-accounting problem. Your conversion rate drops, your infrastructure costs rise, and your actual paying users subsidize the abuse.
• Reputation damage: High bounce rates and spam complaints from invalid addresses hurt your sender reputation. Eventually, your real users stop receiving your emails too.

The maintenance trap

Here's the part that catches most teams off guard: email validation isn't a build-once problem. It's an ongoing maintenance commitment.

Disposable email services actively work to stay ahead of blocklists. They register new domains, use legitimate-looking TLDs, and rotate faster than any manually maintained list can keep up. The moment you think you've blocked them all, a new batch appears.

This is the same dynamic we wrote about with static IP blocklists - a static list decays rapidly. The same is true for email domains. A list you downloaded three months ago is already missing thousands of new disposable domains. If you want to see how comprehensive coverage looks, check our free sample data.

What a maintained database gives you

Instead of building and maintaining your own blocklist, you can use a database that's updated monthly and covers the full scope of the problem:

1. 150,000+ disposable email domains - not 3,000 from a GitHub repo, but comprehensive coverage that's verified and updated every month.
2. New domains caught regularly - new disposable email services get added with each monthly release, not whenever someone notices and opens a pull request.
3. No external data sharing - the database runs on your server. Your signup flow doesn't slow down waiting for a third-party API call. If the vendor's API goes down, your signups don't break.
4. One flat cost - no per-lookup fees that scale with your signups. Whether you validate 100 or 100,000 emails a day, it's the same price.

Build the logic, outsource the data

The SaaS founder who inspired this post made the right call - they recognized email abuse as a real problem and built checks into their platform. The logic they wrote (alias counting, profanity filtering, blacklist checking) is specific to their product and makes sense to own.

What doesn't make sense to own is the underlying data. Maintaining a list of 150,000+ disposable domains, tracking new ones regularly, verifying which ones are still active - that's a dedicated data operation. It's the kind of thing that should come from a dedicated source that's updated monthly and verified, not from a community-maintained list that's six months stale.

That's exactly what we built AntiProxies for. You own your validation logic. We keep the data current. Your signup flow stays clean, your metrics stay honest, and your engineering team stays focused on your actual product. For a deeper look at how throwaway email providers operate, see our post on how disposable email services work. To understand how disposable emails combine with proxy networks to enable large-scale fraud, read how disposable emails and proxies work together. Also consider that email aliases and SPF/DKIM authentication are closely related signals for your validation logic. For technical implementation, see our disposable email detection page.