Three Couryo tools are open to anyone, with or without an account: verify an email address, check a domain's email setup and explain an SMTP error in plain words. They are also on the site, under Tools, and on the MCP.

None of them sends email or opens an SMTP connection to anyone's server: only DNS lookups and rules. Verified addresses are not stored.

Limits#

Without a key, the limit is per IP, per day. With any Couryo key (Authorization: Bearer ck_..., a free account's included), the limit belongs to the account and is much higher.

Tool	Without a key (per IP)	With a key (per account)
`GET /v1/tools/verify-email`	50 a day	2,000 a day
`GET /v1/tools/check-domain`	20 a day	300 a day
`POST /v1/tools/explain-smtp`	50 a day	2,000 a day

Answers carry RateLimit-Limit and RateLimit-Remaining. Above the limit the answer is 429 with the code tool_limit_reached and a Retry-After header. The calls accept CORS from any origin, so they work straight from the browser.

Verify an email#

Terminal

curl "https://api.couryo.com/v1/tools/verify-email?email=ana@hotmail.con"

JSON

{
  "email": "ana@hotmail.con",
  "verdict": "invalid",
  "reason": "typo",
  "did_you_mean": "ana@hotmail.com",
  "checks": {
    "syntax": true,
    "domain": "hotmail.con",
    "receives_email": "no",
    "disposable": false,
    "role": false,
    "free_provider": false
  },
  "mailbox_checked": false,
  "message": "This looks like a typo. Did you mean ana@hotmail.com?"
}

Field	What it is
`verdict`	`valid` (right format and the domain receives email), `risky` (likely typo, temporary inbox or no DNS answer) or `invalid` (wrong format, the domain does not receive email or a sure typo of a big provider, such as `gmail.com.br`)
`reason`	the main reason: `ok`, `bad_syntax`, `no_mail_server`, `typo`, `disposable`, `role` or `dns_unavailable`
`did_you_mean`	the fixed address when the domain looks like a typo of a big provider (`gmial.com`, `hotmail.con`, `gmail.co`)
`checks.receives_email`	`yes` when the domain has an MX (or an address record without MX, as RFC 5321 says), `no` when it has none or publishes a null MX, `unknown` when DNS did not answer
`checks.disposable`	temporary inbox service
`checks.role`	shared mailbox, such as `info@` or `sales@`
`checks.free_provider`	Gmail, Outlook, Yahoo, iCloud and other free providers
`mailbox_checked`	always `false`: the mailbox itself is never probed

Use it in your app's sign-up to catch the typo before sending the confirmation. An invalid address would surely bounce; a risky one deserves a "did you mean...?" on screen.

Check a domain#

Terminal

curl "https://api.couryo.com/v1/tools/check-domain?domain=example.com"

It takes a domain, a URL or an address (only the domain is used). The optional dkim_selector looks up one DKIM selector besides the common ones (it is the s= of the DKIM-Signature header).

Field	What it is
`score` and `grade`	0 to 100 and A to F; SPF, DKIM and DMARC weigh 25 each, MX 15 and MTA-STS, TLS-RPT and BIMI the rest
`summary`	a one-sentence summary
`mail_provider`	who receives the domain's email, from the MX (Google Workspace, Microsoft 365...)
`spf_lookups`	how many DNS lookups the SPF needs (the limit is 10)
`dkim_selectors_found`	the selectors where a DKIM key was found
`checks[]`	one item per check (`mx`, `spf`, `dkim`, `dmarc`, `mta_sts`, `tls_rpt`, `bimi`), with `status` (`ok`, `warning`, `error`, `missing` or `info`), `title`, `detail`, `records` (what DNS answered), `fix` and `suggested_record`, a record ready to copy when there is one

The answer is cached for 10 minutes: after fixing your DNS, wait a little before checking again.

Explain an SMTP error#

Terminal

curl -X POST https://api.couryo.com/v1/tools/explain-smtp \
  -H "Content-Type: application/json" \
  -H "Accept-Language: en" \
  -d '{ "message": "550 5.7.515 Access denied, sending domain does not meet the required authentication level" }'

Field	What it is
`code` and `enhanced_code`	the basic code (`550`) and the enhanced one (`5.7.515`); in an expired message, the reason inside wins (for example `452 4.2.2`, mailbox full)
`kind`	`hard` (the address does not work), `soft` (temporary), `policy` (a provider rule, not the address) or `unknown` (no code in the text)
`provider`	the provider named in the text, when there is one
`explanation` and `next_step`	what happened and what to do, in English or Portuguese following `Accept-Language`
`retry`	whether sending again later can work without changing anything

On the MCP#

The same tools, plus the SPF and DMARC generators, live in two places:

Public server, no login: https://mcp.couryo.com/tools. Any agent can use it, with the per-IP limit. Claude Code: claude mcp add --transport http couryo-tools https://mcp.couryo.com/tools.
Main server (https://mcp.couryo.com, with OAuth or a key): available to every account, the free one included, with the account limit.

Tool	What it does
`verify_email`	verifies an address
`check_domain_dns`	checks a domain's SPF, DKIM, DMARC, MX, MTA-STS, TLS-RPT and BIMI
`explain_smtp_error`	explains an error or bounce
`generate_spf`	builds an SPF record
`generate_dmarc`	builds a DMARC record

See also MCP and agents and the SPF, DKIM and DMARC guide.

Free tools (API and MCP)

Limits#

Verify an email#

Check a domain#

Explain an SMTP error#

On the MCP#