Firebase powers a massive number of apps, from mobile-first products to web SaaS. But Firebase's built-in email capabilities are limited to Firebase Auth emails (verification, password reset). For everything else, you need to connect an external email tool.

The good news: Firebase's Cloud Functions and Firestore triggers make it relatively straightforward to connect to any email platform. The question is which email tool best complements a Firebase-based application.

How Firebase + Email Integration Works

Firebase gives you several connection points:

Firebase Auth triggers: Cloud Functions that fire on user creation, deletion, and sign-in
Firestore triggers: Cloud Functions that fire when documents are created, updated, or deleted
Cloud Functions (HTTP): Custom endpoints that can call email APIs
Firebase Extensions: Pre-built integrations (some email tools have extensions)

The most common pattern: Firestore triggers that call your email tool's API via Cloud Functions.

The 6 Best Options

1. Sequenzy

Best for: SaaS on Firebase wanting lifecycle email automation

Sequenzy's event tracking API works well with Firebase Cloud Functions. When users take actions in your Firebase app, fire events to Sequenzy and trigger lifecycle sequences. The transactional API handles system emails (welcome, receipts) while the marketing side handles campaigns and automations.

Pricing: From $29/month Integration: Event API via Cloud Functions Pros: Lifecycle automation, AI sequences, transactional + marketing Cons: No Firebase Extension, requires Cloud Function setup

2. SendGrid

Best for: Firebase users wanting the easiest built-in integration

SendGrid has a Firebase Extension that makes integration nearly plug-and-play. Install the extension, configure your SendGrid API key, and you can trigger emails from Firestore document changes. It's the most "official" email integration for Firebase.

The extension handles both transactional (triggered by Firestore writes) and basic campaign email. For Firebase users who want minimal setup, it's the path of least resistance.

Pricing: Free for 100 emails/day, from $19.95/month Integration: Firebase Extension (native) Pros: Official Firebase Extension, minimal setup, mature platform Cons: Extension has limitations, platform feels dated, pricing is confusing

3. Resend

Best for: Modern developers wanting clean Firebase email integration

Resend doesn't have a Firebase Extension, but the API integrates cleanly with Cloud Functions. A few lines of code in a Cloud Function gives you full access to Resend's sending capabilities. The DX is excellent, and React Email templates work well for Firebase web apps.

Pricing: Free for 100 emails/day, from $20/month Integration: API via Cloud Functions Pros: Best DX, React Email, clean API, affordable Cons: No Firebase Extension (requires Cloud Function code), marketing features maturing

4. Customer.io

Best for: Technical teams building complex event-driven email on Firebase

Customer.io's event model maps naturally to Firebase's event-driven architecture. Forward Firestore changes and Auth events to Customer.io and build sophisticated email workflows. For apps with complex user journeys, the flexibility is unmatched.

Pricing: From $100/month Integration: API via Cloud Functions Pros: Most powerful automation, event-driven, multi-channel Cons: Expensive, complex, requires significant setup

5. Mailgun

Best for: High-volume Firebase apps needing affordable sending

Mailgun has a Firebase Extension for basic email sending from Firestore. If your Firebase app sends high volumes of email and cost is a priority, Mailgun's infrastructure-level pricing is attractive.

Pricing: Free trial, from $35/month Integration: Firebase Extension available Pros: Affordable at volume, Firebase Extension, infrastructure-level control Cons: Not a marketing platform, basic features, limited automation

6. Postmark

Best for: Firebase apps prioritizing transactional email deliverability

Postmark via Cloud Functions gives you the best transactional email deliverability available. For Firebase apps where email reliability is critical (fintech, healthcare, B2B), Postmark ensures your emails reach inboxes.

Pricing: From $15/month Integration: API via Cloud Functions Pros: Best deliverability, fast delivery, excellent for transactional Cons: Marketing features are basic, no Firebase Extension

Firebase Integration Pattern

The standard approach for any email tool:

User action in app -> Firestore document change -> Cloud Function trigger -> Email tool API call

Example Cloud Function (Node.js):

Listen for Firestore writes to a collection (e.g., users, orders, subscriptions)
Extract relevant data from the document
Call your email tool's API to add a subscriber, trigger an event, or send an email

This pattern works with any email platform that has an API, which is essentially all of them.

FAQ

Should I use Firebase Auth's built-in email or a custom provider? For basic auth flows (verification, password reset), Firebase Auth's built-in email is fine to start. For better deliverability and branding, configure a custom SMTP provider.

Can I use Firebase Extensions for email marketing? Firebase Extensions handle basic transactional email sending (SendGrid, Mailgun). For marketing automation, sequences, and campaigns, you'll need to integrate via Cloud Functions and API calls.

How do I handle email unsubscribes with Firebase? Store unsubscribe preferences in Firestore and check them before sending. Most email tools also handle unsubscribe management on their end, but syncing the status back to your Firebase database keeps things consistent.

Is Cloud Functions the only way to integrate? It's the most common. You can also use client-side API calls (less secure), or third-party services like Zapier. Cloud Functions is recommended because it keeps your API keys server-side.