Use Firebase Authentication when the goal is account creation and sign-in; use Amazon SES when the product needs a true numeric email OTP with custom rules, branding, and audit trails. Amazon SES lists email sending at $0.10 per 1,000 emails, before extras such as attachments or incoming mail processing [source: AWS]. That sounds almost free, but the API is only one piece. The real question is who owns the verification logic.

TLDR: Firebase is the faster free path for standard email verification, but it is usually link-based rather than a classic 6-digit OTP flow. Amazon SES is better for sending codes such as 483921, yet you must build code generation, expiry, rate limits, and storage yourself. For example, a SaaS onboarding 20,000 users per month with 1.3 email attempts per user would send about 26,000 verification emails; SES mail cost is tiny, but engineering time is the real bill. If your team has one backend developer and wants to launch this week, Firebase usually wins.

What “email OTP free service API” really means

An email OTP service does four jobs. It creates a code, sends it, checks it, and blocks abuse. The email sender is only the loudest part.

• Generate: Create a short code, often 6 digits.
• Store: Save a hashed version with an expiry time.
• Send: Deliver the code by email.
• Verify: Match the user input and reject old or repeated attempts.
• Protect: Add rate limits, IP rules, device checks, and logs.

This is where Firebase and SES split. Firebase Authentication is an identity product. Amazon SES is an email delivery product. One verifies users. The other sends messages. Both can be used for email verification, but they are not the same kind of tool.

Firebase Authentication: quickest path for app sign-in

Firebase Authentication is attractive because it removes a lot of boring work. You can set up email and password sign-in, send verification emails, reset passwords, and manage users from a ready-made console. Firebase also supports email link sign-in, which lets users access an app through a secure link instead of a password [source: Firebase].

For many apps, that is enough. A marketplace, community app, internal dashboard, or beta SaaS can use Firebase and avoid building a full authentication service. The SDKs are clean. The client-side setup is fast. User management is already there.

The annoying part is the wording. People search for “Firebase email OTP,” then expect a ready-made endpoint that sends 123456 and verifies it. Firebase Authentication is not built around that pattern for email. It is built around authentication flows, action links, and provider-based sign-in. If your product manager insists on a 6-digit code screen, you may need extra pieces.

With Firebase, a custom numeric email OTP often means:

• Cloud Functions to generate and verify codes.
• Firestore or another database to store hashed codes.
• An email provider to send the message.
• Rules to stop code spam and repeated guessing.

At that point, Firebase is still useful, but it is no longer a one-click OTP service. It becomes your backend base.

Amazon SES: flexible, cheap, and very hands-on

Amazon SES is excellent at sending email. It is built for scale, transactional messages, and deliverability controls. If you want to send a message that says, “Your login code is 483921,” SES can do that through an API or SMTP.

You can control the subject line, sender domain, reply address, templates, tracking, and headers. You can also connect it to Lambda, API Gateway, DynamoDB, Cognito, or your own server. That control is the main reason teams pick SES for OTP flows.

But SES does not verify codes for you. It does not decide whether a code should expire in 5 minutes or 15. It does not know if one IP address requested 40 codes in a minute. You must build that logic.

Expect to waste time on setup if you have never used SES before. Domain verification, DNS records, SPF, DKIM, DMARC, sandbox removal, bounce handling, and complaint rates all matter. None of these are hard in isolation. Together, they can turn a “quick email API task” into a half-day debugging session.

Firebase vs Amazon SES at a glance

Feature	Firebase Authentication	Amazon SES
Best use	App sign-in and account verification	Sending custom OTP emails
OTP style	Mostly link-based unless customized	Any format you build
Backend required	Not for standard auth	Yes, for OTP generation and checks
Branding control	Moderate	High
Abuse controls	Built into auth flows	You design them
Speed to launch	Fast	Medium to slow

Which one is more “free”?

Firebase feels free because you can start without wiring a mail system. For common email verification, that is a huge win. You are not paying per verification email in the same visible way, and you get user records, sessions, and SDK support.

SES feels free because email delivery costs are very low. Sending tens of thousands of OTP emails may cost less than a coffee. Still, low sending cost does not include the work of building secure OTP logic. If an engineer spends two days creating a safe system, that is the larger cost.

There is also a risk cost. A weak OTP flow can be abused. Attackers can spam inboxes, enumerate accounts, or brute-force codes. A 6-digit code has only 1,000,000 possible values. That sounds large, but without rate limits, it is not enough. A secure implementation should limit attempts per user, per IP, and per device fingerprint.

Recommended architecture for email OTP with SES

If you choose SES, keep the flow simple and strict:

1. User enters an email address.
2. Your backend creates a random 6-digit code.
3. The backend stores only a hashed code, not the raw code.
4. Set expiry to 5 to 10 minutes.
5. Send the email through SES.
6. User submits the code.
7. Backend checks the hash and attempt count.
8. On success, create the account or session.

Do not store OTPs in plain text. Do not reuse codes. Do not allow unlimited resends. A good resend rule is one email every 30 to 60 seconds, with a daily cap. For sensitive products, add risk scoring or CAPTCHA after repeated requests.

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When Firebase is the smarter choice

Pick Firebase if your main need is user authentication, not email infrastructure. It is ideal for MVPs, mobile apps, student projects, community platforms, and admin portals. Your team gets user sessions, password reset, email verification, and client SDKs with far less code.

Firebase is also a good choice when your design can accept email links. A link feels natural on mobile. Tap, return to the app, done. No copying a code from Mail to Chrome while the app reloads and loses state. We have all seen that mess.

When Amazon SES is the smarter choice

Pick SES if the OTP experience must be fully custom. Banks, B2B SaaS tools, healthcare portals, and enterprise apps often need exact message wording, branded templates, logs, regional controls, and custom security rules.

SES also fits well if you already run on AWS. Lambda can generate the code. DynamoDB can store the hash and expiry. API Gateway can expose the endpoints. CloudWatch can track failures. It is clean once built, but it is still your system to maintain.

Final recommendation

For most small teams, Firebase Authentication is the better free starting point. It solves the bigger problem: authenticating users safely. If a verification link is acceptable, ship with Firebase and spend your time on the product.

For teams that need true email OTP codes, Amazon SES is the better sender. Just be honest about the missing parts. SES sends the email. You build the OTP service. That trade is worth it when control matters more than speed.