What is an email API?

You want your app to send a confirmation, read the reply, and keep both in the same conversation. The old way means stitching SMTP for outbound, IMAP for inbound, a MIME parser in the middle, and a fresh round of OAuth for every provider you support. That’s four moving parts before you write a line of product code, and each one breaks differently.

An email API replaces that stack with HTTPS calls and JSON. You send with one request, read with another, and the same integration works whether the mailbox lives on Gmail, Outlook, or a self-hosted IMAP server. This page defines what an email API is, what it does, how it differs from the raw protocols, and where to go deeper.

What is an email API and how does it work for sending and receiving email?

An email API is an HTTP interface that sends, reads, parses, and syncs email through JSON requests instead of raw mail protocols. You send a POST to deliver a message and a GET to list an inbox, both over HTTPS on port 443. The API holds the provider connection, runs OAuth, and returns structured JSON for the full lifecycle.

Internally, the API does the protocol work you’d otherwise own. To send, it routes your POST /v3/grants/{grant_id}/messages/send through the connected provider’s outbound servers, so deliverability inherits that mailbox’s warmed reputation. To read, it syncs the mailbox and exposes messages through GET /v3/grants/{grant_id}/messages, already parsed into fields like subject, from, body, and thread_id. Real-time changes arrive through webhooks: register an endpoint at /v3/webhooks, and a message.created event fires when new mail lands, with payloads capped at 1 MB. That’s send, read, parse, and sync behind four HTTPS surfaces instead of three separate protocols.

How does an email API differ from SMTP and IMAP?

SMTP and IMAP are single-purpose transport protocols: SMTP only sends mail, IMAP only reads it, and neither returns JSON or handles OAuth. An email API sits one layer above both. It wraps sending, reading, and parsing behind HTTPS endpoints, returns structured JSON, and manages authentication, so a single integration covers what SMTP and IMAP do separately.

The protocols themselves are decades old. SMTP, defined in RFC 5321, handles the outbound hop and stops there. IMAP (RFC 3501) is a stateful, socket-level protocol that has read one mailbox at a time since the 1990s. To build a two-way feature with raw protocols, you open TCP connections, parse FETCH responses by hand, assemble MIME strings to send, and store per-mailbox credentials. Cloud hosts complicate this further: AWS and Google Compute Engine block outbound port 25 by default to curb spam. An email API runs over HTTPS on 443, which is never blocked, and collapses both protocols into one schema. For the full breakdown, see SMTP vs a REST email API and IMAP vs an email API.

SMTP, IMAP, and a unified email API compared

The table below compares raw SMTP and IMAP against a unified email API across the dimensions that decide an integration: direction, transport, auth, and provider coverage. The protocols win on simplicity for one narrow job each. A unified API wins anywhere the app needs structure, two-way mail, or more than one provider.

Dimension	SMTP	IMAP	Unified email API
Direction	Send only	Read only	Send, read, and sync
Transport	TCP 25 / 587 / 465	TCP 143 / 993	HTTPS on 443, never blocked
Response format	Numeric reply codes	Untyped `FETCH` data	Structured JSON
Auth	Per-mailbox password	Per-mailbox password	OAuth through `/v3/connect/auth`
Real-time updates	None	`IDLE` polling	Webhooks at `/v3/webhooks`
Multi-provider	Per-host config	Per-host config	One schema across 6 providers

What does a unified multi-provider email API mean?

A unified email API puts one schema and one OAuth flow in front of every provider, so you write your integration once and reach all of them. Connect a Gmail, Microsoft 365, Yahoo, iCloud, generic IMAP, or Exchange account, and each returns the same JSON shape. You read every mailbox through GET /v3/grants/{grant_id}/messages regardless of what’s behind it.

The alternative is one integration per provider. Gmail uses the Gmail API with users.messages.list; Microsoft uses Graph with GET /me/messages. Each has its own OAuth project, ID format, rate limits, and quirks, so supporting two providers means maintaining two unrelated codebases. A unified layer collapses that into a single code path. The connection is represented by a grant, an authenticated account you reference by grant_id on every call, so adding iCloud or Exchange later costs no new integration. See the unified inbox guide for how a single feed merges multiple accounts.

What APIs can help me integrate email services into my application?

Three categories of API cover email integration. Transactional APIs (like SendGrid or Postmark) send app-generated mail at volume but don’t read inboxes. Provider APIs (Gmail API, Microsoft Graph) give deep single-provider access. A unified email API like the Nylas Email API sends and reads across all 6 providers through one schema, which fits apps that work with users’ own mailboxes.

The right pick depends on direction and provider count. If you only blast one-way notifications and never read replies, a transactional sender is enough and lighter to run. If your product lives entirely inside one ecosystem and needs provider-only surfaces like Teams or Drive, the native API is the only option. The moment you read users’ mail across more than one provider, the unified model removes the per-provider OAuth, ID mapping, and retry logic. The send call below delivers a message from a connected account through POST /v3/grants/{grant_id}/messages/send, routing it through that mailbox’s own infrastructure across all 6 supported providers.

curl --request POST \
  --url 'https://api.us.nylas.com/v3/grants/<NYLAS_GRANT_ID>/messages/send' \
  --header 'Authorization: Bearer <NYLAS_API_KEY>' \
  --header 'Content-Type: application/json' \
  --data '{
    "to": [{ "name": "Leyah Miller", "email": "[email protected]" }],
    "subject": "Your account is ready",
    "body": "Welcome aboard. Your workspace is set up and waiting."
  }'

Which email API is best for developers building SaaS applications?

The best email API for a SaaS app depends on whether you touch users’ own mailboxes. If your product reads and sends from customer inboxes across Gmail, Outlook, and others, a unified API across all 6 providers fits best because one OAuth flow covers every customer. If you only send app-generated mail one direction, a transactional sender is the right call.

For SaaS that reads customer mail, the operational wins compound. The read call below lists the 20 most recent unread messages from a connected inbox through GET /v3/grants/{grant_id}/messages, returning the same JSON whether the mailbox is Google or Exchange. It accepts subject, from, unread, received_after, and thread_id as query parameters, so you filter without parsing raw headers. To thread a reply into an existing conversation, pass reply_to_message_id on the send call, and the API sets the In-Reply-To and References headers for that provider.

The honest tradeoff: if you genuinely send one-way mail to one provider and never read a reply, wiring up a unified API with OAuth grants is more than the job needs. A plain SMTP relay or a transactional sender is the lighter choice there. The unified model earns its place once you read inboxes, support more than one provider, or need webhooks for real-time updates.

What’s next

SMTP vs a REST email API to compare the sending side protocol by protocol
IMAP vs an email API to compare the reading side in depth
Build a unified inbox to merge multiple provider accounts into one feed
Getting started with Nylas to create a project, connector, and your first grant