> ## Documentation Index
> Fetch the complete documentation index at: https://docs.relaycli.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Key concepts

> How modern e-mail really works and the pieces we build on

<Note>Why read this? You don’t need to memorize every RFC to use Relay, but a 5-minute tour of the landscape helps you debug faster — and impress friends at parties.</Note>

## Standards & Governance

### IETF

Internet Engineering Task Force – the community that writes the open specifications powering e-mail, HTTP, TLS, etc.
All core mail specs are “RFCs” (Request for Comments) published by the IETF.

### RFC

A numbered document that defines a protocol. Example: RFC 5322 specifies e-mail headers.
Every server and client that follows the same RFC can inter-operate – that’s why you can fetch Gmail via Apple Mail.

## Anatomy of an email

* Headers: key-value pairs describing sender, recipients, threading
* Body: one or more <Tooltip tip="Multipurpose Internet Mail Extensions">MIME</Tooltip> “parts” (plain text, HTML, PDF, images)

```shell email example theme={null}
Headers
  From:       alice@example.com
  To:         bob@example.org
  Subject:    Weekly sync?
  Message-ID: <CA123@example.com>
  In-Reply-To:<B7@example.org>

Body (plain text or HTML + attachments)
```

## Email Protocols

* <Tooltip tip="Internet Messaging Access Protocol">IMAP</Tooltip> (RFC 3501) — read & organize mail.
* IDLE (RFC 2177) — real-time notifications.
* <Tooltip tip="Post Office Protocol">POP3</Tooltip> (RFC 1939) — download and delete mail.
* <Tooltip tip="Simple Mail Transfer Protocol">SMTP</Tooltip> (RFC 5321) — send mail.
* <Tooltip tip="JSON Meta Application Protocol">JMAP</Tooltip> (RFC 8620) — modern JSON read/send.

## From Server to App: Who Does What?

| Layer               | Acronym | Example software     | What it does                                                             |
| ------------------- | ------- | -------------------- | ------------------------------------------------------------------------ |
| Mail Transfer Agent | MTA     | Postfix, Exim        | Routes SMTP traffic between domains.                                     |
| Mail Delivery Agent | MDA     | Dovecot, Courier     | Saves incoming mail to the user’s mailbox.                               |
| Mail User Agent     | MUA     | Outlook, Thunderbird | <Tooltip tip="Graphical User Interface">GUI</Tooltip> that people click. |
| Relay               | -       | (open-core)          | Sits between MUA and IMAP, giving dev-friendly JSON + webhooks.          |

## Providers vs. Clients

* Provider = organization hosting an MTA/MDA (Gmail, Outlook, Proton, your own VPS).
* Client = anything that logs in to read or send mail (iOS Mail, Superhuman, your script).

Relay treats every provider the same as long as it offers IMAP/SMTP. Your code talks to a single /messages endpoint.

## Email APIs vs. Delivery APIs

Both provide programmable access to email-related operations.

| Category         | Examples              | Focus                                         |
| ---------------- | --------------------- | --------------------------------------------- |
| Delivery APIs    | Resend, SendGrid, SES | High-volume outbound (newsletters, receipts). |
| Interaction APIs | Relay                 | Read, search, tag, reply, build full apps.    |

Use both together: Resend for blasts, Relay for smart follow-ups.

## Key Terminology in Relay

| Term          | Short definition                                           |
| ------------- | ---------------------------------------------------------- |
| Email account | IMAP/SMTP credential set you connect                       |
| Message       | A single e-mail, defined by its Message-ID or IMAP UID.    |
| Thread        | All messages sharing References/In-Reply-To headers        |
| Label         | User-defined tag saved server-side; survives new messages. |

## Ready?

Head back to [Quick-start](/documentation/getting-started/quickstart).
We can’t wait to see what you’ll build!

Happy hacking — and welcome to programmable e-mail.