What an AI sales agent must know before acting

The cheapest failure to prevent is mail to a domain that accepts none. A domain can publish a null MX record, a single MX with preference 0 and a lone dot as the exchange, to say it takes no mail. A domain that publishes it must not publish any other MX record ¹. Without an MX record at all, senders fall back to the domain address records and keep retrying for about a week before giving up ¹. An agent that writes and schedules a message to such a domain spends tokens and sender reputation for nothing.

Our email domain check reads public DNS for the domain behind an address. It reports MX records, a null MX flag, an implicit MX flag for the address record fallback, the SPF and DMARC records it finds, whether the domain is a known disposable provider or a free mailbox provider, and whether the local part is a role mailbox. It never opens a connection to the mail server, so the field smtp_checked is always false and a good result does not prove that a particular mailbox exists.

Role mailboxes are worth separating from people. RFC 2142 lists the conventional names: info, marketing, sales and support for business functions, abuse, noc and security for network operations, and postmaster, hostmaster and webmaster for services ². A message to one of them reaches a shared queue, so an agent that writes as if to a named person should route it differently or skip it.

The phone lookup parses a number against the numbering plan and returns the E.164 form, national and international formatting, the country, an area code region for North American numbers and a number type such as mobile, fixed line, voip or toll free. The type comes from libphonenumber metadata, which assigns a type whenever feasible and distinguishes fixed line, mobile, toll free, premium rate, shared cost, VoIP and others ³. Its own documentation separates a quick length check from full validation using length and prefix information ³.

This check is about the numbering plan only. It does not contact a carrier, so it cannot say that a number is switched on, ported or reachable, and the response says so in a notes field. Use it to reject numbers that cannot exist and to pick the right channel, not to promise delivery.

Two requests give the agent both signals. The bodies below are the ones the API accepts; each costs one call and each returns a small JSON record the gate can read.

1curl -X POST https://YOUR_API_HOST/v1/email-domain-check \
2  -H "Authorization: Bearer $LOOKVALID_API_KEY" \
3  -H "Content-Type: application/json" \
4  -d '{"email":"sales@example.com"}'

1curl -X POST https://YOUR_API_HOST/v1/phone \
2  -H "Authorization: Bearer $LOOKVALID_API_KEY" \
3  -H "Content-Type: application/json" \
4  -d '{"phone":"+14155552671"}'

The function below takes the data object from each reply and returns one decision. Keep it in code, not in the prompt, so the model cannot talk its way past it.

1type EmailData = { deliverability_signal: 'good' | 'risky' | 'undeliverable'; disposable: boolean; role_account: boolean | null };
2type PhoneData = { number_type: string } | null; // null when the lookup found no valid number
3​
4export function nextStep(email: EmailData, phone: PhoneData): 'send' | 'review' | 'skip' {
5  if (email.deliverability_signal === 'undeliverable' || email.disposable) return 'skip';
6  if (email.deliverability_signal === 'risky' || email.role_account) return 'review';
7  if (phone && ['toll_free', 'premium_rate', 'voip'].includes(phone.number_type)) return 'review';
8  return 'send';
9}

• Call the domain check when a contact enters your system, and again before a sequence starts, since DNS changes.
• Call the phone lookup once, store the E.164 value, and reuse it.
• Log every gate decision with the field that caused it, so you can explain a skipped contact later.

The same lookups are available to AI clients as tools over MCP, which suits an agent that decides for itself when to check. Start with the quickstart, then add the email domain check in front of your first send.