Gmail filter sender not in contacts: route unknown senders safely.

Free Apps Script: label senders not in Google Contacts

This script labels inbox mail from anyone who is not in your Google Contacts or Other contacts. It leaves the mail in the inbox; set ARCHIVE to true to archive it as well.

// Screen Gmail senders who are not in your Google Contacts.
// Runs in your own Google account. Nothing is sent to KeepKnown.
const LABEL_NAME = 'Not in contacts';
const ARCHIVE = false; // true: label and archive. false: label only.
const SEARCH = 'in:inbox newer_than:1d';

function screenInbox() {
  const known = knownAddresses_();
  const label = GmailApp.getUserLabelByName(LABEL_NAME) || GmailApp.createLabel(LABEL_NAME);
  GmailApp.search(SEARCH, 0, 100).forEach(function (thread) {
    const labelled = thread.getLabels().some(function (existing) {
      return existing.getName() === LABEL_NAME;
    });
    const fromKnownSender = thread.getMessages().some(function (message) {
      return known.has(addressOf_(message.getFrom()));
    });
    if (labelled || fromKnownSender) return;
    thread.addLabel(label);
    if (ARCHIVE) thread.moveToArchive();
  });
}

function knownAddresses_() {
  const known = new Set([Session.getActiveUser().getEmail().toLowerCase()]);
  GmailApp.getAliases().forEach(function (alias) {
    known.add(alias.toLowerCase());
  });
  let pageToken;
  do {
    const page = People.People.Connections.list('people/me', {
      personFields: 'emailAddresses',
      pageSize: 1000,
      pageToken: pageToken,
    });
    (page.connections || []).forEach(function (person) {
      addEmails_(known, person);
    });
    pageToken = page.nextPageToken;
  } while (pageToken);
  do {
    const page = People.OtherContacts.list({
      readMask: 'emailAddresses',
      pageSize: 1000,
      pageToken: pageToken,
    });
    (page.otherContacts || []).forEach(function (person) {
      addEmails_(known, person);
    });
    pageToken = page.nextPageToken;
  } while (pageToken);
  return known;
}

function addEmails_(known, person) {
  (person.emailAddresses || []).forEach(function (email) {
    known.add(String(email.value).toLowerCase());
  });
}

function addressOf_(from) {
  const match = from.match(/<([^>]+)>/);
  return (match ? match[1] : from).trim().toLowerCase();
}

The script checks your Google Contacts and Other contacts (people you have emailed) on a timer, so new mail can sit in the inbox for a few minutes. Apps Script has daily quotas and a 6-minute limit per run, and you maintain the code yourself. To let a sender through, add them to Google Contacts; mail you move back keeps its label and is not archived again.