The glossary

35 terms from email APIs, explained in plain words, with where they matter in iResendly.

A

API key: A secret string that identifies your account to an API. iResendly keys start with irs_, can have full or sending-only access, and should only be used on your server.Authentication and API keys
Attachment: A file sent along with an email. Through an API it's usually passed as base64 content or as a URL the service downloads.Send email

Base64: A way of writing binary data, such as a file, using only plain text characters so it can travel inside JSON. It makes the data about a third larger.Send email
Batch send: Sending many separate emails in a single API request instead of one request per email. iResendly's batch endpoint takes up to 100 emails at once.Send email
Bearer token: A credential sent in the Authorization header as Bearer <token>. Whoever holds, or bears, it is let in, so it must be kept secret.Authentication and API keys
Bounce: An email the receiving server wouldn't accept. A hard bounce is permanent, such as an unknown address; a soft bounce is temporary, such as a full mailbox. iResendly reports permanent bounces as email.bounced and delays as email.delivery_delayed.Webhooks
Broadcast: A marketing email sent to a group of contacts at once, such as a newsletter. In iResendly, a broadcast goes to a segment, and an unsubscribe link is added if the email has none.Broadcasts

Complaint(Spam complaint): A recipient marking your email as spam, reported back to the sender by the mailbox provider. Complaints hurt sender reputation; in iResendly they suppress the address and unsubscribe the contact.Deliverability and account health
CORS(Cross-Origin Resource Sharing): The browser rules that decide whether a web page may call an API on another domain. The iResendly API is meant for servers, because requests carry a secret key, so calls from browsers are blocked.Developer API
Cursor pagination: Fetching a long list a page at a time by passing the id of the last item you received, rather than a page number. iResendly list endpoints take limit, after and before, and say whether there's more with has_more.Emails reference

DKIM(DomainKeys Identified Mail): A signature added to each email and checked against a public key in the sending domain's DNS, proving the email came from that domain unchanged. Verifying a domain with iResendly includes a DKIM record you publish.Domains

Endpoint: One address in an API that does one job, named by its HTTP method and path, such as POST /emails to send an email.API reference
Event: A record that something happened, such as an email being delivered, opened or bounced. Webhooks deliver events to your server as they happen.Webhooks

HMAC(Hash-based message authentication code): A code made by hashing a message together with a shared secret. Only someone with the secret can produce the same code, so it proves who sent the message and that it wasn't changed.Webhooks
HTTP status code: The three-digit number at the start of every API response. 2xx means success, 4xx means something is wrong with the request, such as a bad key or invalid field, and 5xx means a problem on the server's side that's worth retrying.Errors

Idempotency key: A unique value you send with a request so that repeating it, for example after a timeout, doesn't do the work twice. The server returns the first response for any repeat with the same key.Idempotency
Inline image(CID image): An image attached to an email and shown inside its HTML by referring to its Content-ID, rather than loaded from a URL.Send email
ISO 8601: The international standard format for dates and times, such as 2026-11-20T09:00:00Z. APIs use it so times are unambiguous across time zones.Send email

JSON: A plain-text format for structured data made of objects, arrays, strings and numbers. Most REST APIs, including iResendly's, send and receive JSON.Developer API

List-Unsubscribe(One-click unsubscribe): Email headers that let mail apps show their own unsubscribe button and unsubscribe the person in one click. Mailbox providers expect them on marketing email; iResendly adds them to broadcasts.Broadcasts

MIME(Multipurpose Internet Mail Extensions): The standard that structures an email into parts, such as an HTML version, a plain-text version and attachments, each with its own content type.

Open and click tracking: Measuring opens with a tiny invisible image and clicks by routing links through a tracking address. In iResendly both are off by default and turned on per domain.Domains

Payload: The data carried in the body of a request or webhook, usually JSON. An iResendly webhook payload has type, created_at and data.Webhooks

Rate limit: The most requests an API accepts in a period of time, such as per second. Going over returns 429 Too Many Requests until the window resets.Rate limits and quotas
REST API: An API built on plain HTTP, where resources such as emails or domains have URLs and you act on them with methods like GET, POST, PATCH and DELETE.Developer API
Retry-After: A response header that tells the client how many seconds to wait before trying again, usually sent with a 429 or 503 response.Rate limits and quotas

Sandbox(Test mode): A separate environment where you can try an API without real effects, such as emails that are accepted but never delivered. Not every service has one; another approach is to send tests to addresses you own.
Scheduled send: Accepting an email now and delivering it at a set time later. Scheduled emails can usually be moved or cancelled until they go out.Send email
Standard Webhooks: An open specification for signing and sending webhooks, with webhook-id, webhook-timestamp and webhook-signature headers. iResendly signs webhooks this way, so standard verification libraries work.Webhooks
Suppression list: Addresses a sender won't email, usually because they hard bounced or complained. iResendly skips suppressed recipients and reports them with an email.suppressed event.Deliverability and account health

Template variables: Placeholders in a saved email template that are filled in with values you pass when sending, such as a name or an order number.Templates
Too Many Requests(429): The HTTP status returned when you've gone over a rate limit or quota. Wait for the time in the retry-after header, then try again.Errors
Transactional email: Email sent because of something one person did, such as a receipt, password reset or sign-in code. Marketing email, by contrast, goes to many people at once and must offer a way to unsubscribe.Send email

Webhook: An HTTP request a service sends to a URL you choose when something happens, so your system learns about events without polling. Your endpoint should reply with a 2xx quickly and do slow work afterwards.Webhooks
Webhook signature: A code sent with each webhook, made from the request and a secret only you and the sender know, so you can check the request is genuine. Verify it against the raw body before trusting the event.Webhooks