Use multiple providers

Route each message to the right provider from one function.

Connect several providers and keep the choice between them in one function: resolveEmailDriver. The rest of your app never mentions a provider.

One resolver, one lookup

In practice, which provider sends a message is a fact your database already knows — which provider is this sender address connected through? So the resolver is a lookup, not a rulebook:

const emailkit = EmailKit({
  emailDrivers: [
    ResendDriver({
      id: "resend",
      apiKey: process.env.RESEND_API_KEY!,
    }),
    GmailDriver({
      id: "gmail",
      clientId: process.env.GOOGLE_CLIENT_ID!,
      clientSecret: process.env.GOOGLE_CLIENT_SECRET!,
    }),
    OutlookDriver({
      id: "outlook",
      clientId: process.env.OUTLOOK_CLIENT_ID!,
      clientSecret: process.env.OUTLOOK_CLIENT_SECRET!,
    }),
  ],
  resolveEmailDriver: async (context) => {
    // Mailbox and domain calls name their driver explicitly ({ emailDriver: "gmail" }),
    // so this resolver only routes sends — anything else falls back to the API driver.
    if (context.operation !== "sendEmail") return "resend";

    // a connected inbox: driver plus its mailbox and saved login
    const connected = await prisma.connectedMailbox.findUnique({
      where: { email: context.message.from.email },
    });
    if (connected) {
      return {
        emailDriver: connected.emailDriver, // "gmail" or "outlook"
        mailbox: connected.mailbox,
        auth: decrypt(connected.auth),
      };
    }

    // everything else sends through the API driver
    return "resend";
  },
});

TypeScript knows your configured IDs and catches invalid ones. Loading the mailbox and login here means no other call site ever handles credentials.

This is the core of the users bring their own email use case — that page shows the onboarding flows around it.

Override for one message

sender.emailDriver skips the resolver when one message must use a specific provider — say, a send that needs a feature only one of them has:

await emailkit.sendEmail({
  from: { email: "news@example.com" },
  to: { email: "reader@example.com" },
  subject: "July newsletter",
  text: "This month's news...",
  sendAt: tomorrow,
  sender: { emailDriver: "resend" },
});

Domain and mailbox methods take the provider at the top level instead:

await emailkit.domains.verify({
  emailDriver: "resend",
  domain: "example.com",
});

Same provider, twice

Driver IDs also let you connect one provider more than once — a Mailgun account in the EU and one in the US, or separate accounts for transactional and marketing mail:

emailDrivers: [
  MailgunDriver({ id: "mailgun-eu", region: "eu", apiKey: euKey }),
  MailgunDriver({ id: "mailgun-us", region: "us", apiKey: usKey }),
],