Migrate transactional email from SendGrid to Postal

To migrate transactional email from SendGrid to Postal, move one message stream at a time: inventory API and SMTP usage, configure Postal and authenticate a sending domain, adapt the application to Postal’s API, verify delivery and bounce handling, then shift traffic gradually with a rollback path. This is not a drop-in endpoint change. Postal is software you operate; you take responsibility for the sending IP, DNS, reputation, database, upgrades, and delivery monitoring.

This guide covers transactional sending, not a complete migration of SendGrid Marketing Campaigns, contacts, suppressions, or templates. Inventory those separately before retiring a SendGrid account. Postal and SendGrid expose different APIs and event formats, so preserve the old path until the new one has passed your acceptance checks.

1. Inventory the existing sending path

Search application code and deployment configuration for SendGrid API calls, SMTP credentials, templates, scheduled jobs, and event webhooks. Record:

• sending domains and From addresses;
• transactional message types and their criticality;
• template IDs, dynamic fields, attachments, and unsubscribe behavior;
• API/SMTP usage by service and environment;
• delivery, bounce, block, and complaint events consumed by the application;
• credentials, rate limits, retry rules, and any provider-specific response handling.

Export or otherwise preserve suppression state and relevant delivery history according to your policy and provider terms. Do not assume a SendGrid template, suppression list, or webhook can be imported directly into Postal. Map each item to a Postal feature or to an application-owned workflow before you begin the cutover.

2. Prepare Postal before changing application traffic

Install Postal and verify the host, MariaDB, TLS, reverse DNS, and mail-server health. See how to install Postal v3. Add and verify the sending domain in Postal, publish the exact DNS records that Postal generates, and test a controlled message from the intended From address.

Warm up and monitor the sending IP according to your actual sending profile. A new or previously used IP may have a different reputation than SendGrid’s shared or dedicated sending infrastructure. Postal does not transfer SendGrid’s reputation to your server. Do not move all transactional traffic at once simply because a single test message arrived.

3. Replace the API integration

SendGrid’s API and Postal’s API have different authentication headers, endpoints, request fields, and response envelopes. Isolate the provider behind a small application interface so each message call has one delivery implementation:

export async function sendTransactionalEmail(message) {
  if (process.env.EMAIL_PROVIDER === "postal") {
    return sendWithPostal(message);
  }
  return sendWithSendGrid(message);
}

This is an architecture sketch, not a complete provider adapter. Implement separate request/response mapping for each provider. For Postal, use the documented /api/v1/send/message endpoint and X-Server-API-Key; check both HTTP status and the JSON status field. Capture Postal’s returned message ID and per-recipient identifiers. See send with Postal’s API from Node.js.

Keep credentials in your secret manager and scope Postal API credentials to the appropriate mail server. Map provider errors into application-level categories only when the source response supports that mapping. Do not claim identical retry or idempotency behavior across providers.

4. Migrate events and delivery state

List every SendGrid event your application consumes. Recreate the required event flow with Postal’s message webhooks and test the mapping. Store provider name and provider message ID with each send record; otherwise, after dual-provider operation, an event may not identify which provider owns the message.

Implement idempotent event handling. Providers can retry webhook delivery, and your application should not send a second receipt, reopen a completed workflow, or duplicate a customer notification when it receives the same event again. Preserve distinct states such as accepted, delayed, delivered, bounced, and failed rather than collapsing every webhook into “sent.” Verify the event’s meaning in Postal’s documentation before translating it into your internal status.

If your application relies on SendGrid marketing campaigns, suppression management, or contact administration, do not assume Postal replaces those systems. Postal is a mail delivery platform; campaign management and consent state may remain in your application or another tool.

5. Cut over gradually

Use an environment variable or server-side routing rule to move one low-risk transactional stream first. Keep a single provider selected for each message. Sending the same message through both systems is not a safe migration test; it can deliver duplicate mail.

Before increasing traffic, compare operational signals you can observe: API acceptance, Postal queue state, delivery events, bounce counts, delays, and representative mailbox checks. These are separate measures. Do not infer inbox placement from a successful API response or from a small test sample.

Increase traffic in controlled steps only when the preceding step meets your predeclared criteria. If Postal’s queue, host, or reputation has a problem, route new messages back to SendGrid while preserving the application send ledger. Avoid replaying uncertain sends until you reconcile whether the original provider accepted them.

6. Keep a rollback and decommission plan

Keep SendGrid credentials and configuration available during the agreed observation period. Document the switch back, the owner who can make it, and how queued or ambiguous sends are handled. Reconcile suppression state before removing the old integration. Only then revoke credentials and remove unused provider configuration.

When MailKite may fit instead

Postal is a reasonable choice when you explicitly want to operate your delivery infrastructure and can own reputation, database, and server maintenance. MailKite is a hosted email API and inbound webhook platform; it can replace some sending and receiving infrastructure, but it is not a Postal-compatible API and does not eliminate the need to migrate message templates, event consumers, or application logic.

Keep Postal for outbound and use MailKite for inbound

You can use both products when you want to keep Postal’s self-hosted outbound delivery but prefer a managed inbound webhook. For example, send application notifications from notify.example.com through Postal, and receive replies at replies.example.com through MailKite. Configure each hostname according to that provider’s DNS instructions. Do not point the same hostname’s MX records at both providers; a domain’s MX configuration determines where inbound mail is delivered.

In the application, keep separate provider adapters and credentials: Postal handles outbound requests and delivery events, while MailKite receives email on its own verified inbound domain and sends signed email.received webhooks to your app. Set Reply-To to an address on the inbound domain only if the recipient’s reply should arrive through MailKite. Verify sender identity and reply routing separately, then test a real controlled round trip. The two APIs do not share message IDs, credentials, or event schemas.

This arrangement is useful when you have an operational reason to retain Postal, but it does not remove Postal’s outbound hosting and reputation work. If you also want MailKite to send, compare the two paths and choose deliberately rather than duplicating each message.

Use the MailKite sending docs for the current SDK contract and receiving docs for inbound setup. For a Postal/MailKite agent-inbox comparison, read Postal for AI agents. Choose a provider per workflow based on ownership, product fit, and your team’s operational capacity.

Sources

• Postal API
• Postal webhooks
• Postal prerequisites
• MailKite sending documentation
• MailKite receiving documentation