Custom SMTP provider support for workflow email sends

Feature request

Feature request

Custom SMTP Provider Support for Workflow Email Sends

Feedback: https://posthog.com/questions/custom-smtp-support

Problem

Currently, workflow email sending only supports a single built-in email provider. Users who have existing email infrastructure (e.g. their own SMTP server or a preferred email service) cannot use it for workflow email sends.

This blocks adoption for teams that want to send workflow emails from their own domain/provider for compliance, deliverability, or branding reasons.

Proposed Solution

Allow users to configure a custom SMTP server as an email provider for workflows.

This would involve:

1. Adding SMTP as a supported provider in the email integration setup
2. Allowing users to configure SMTP host, port, credentials, and TLS settings
3. Domain verification flow for custom SMTP providers

Why this matters

• Unblocks users who want to migrate their workflows to PostHog but need custom email infrastructure
• Gives teams control over email deliverability and sender reputation
• Supports compliance requirements around email sending

Debug info

- [ ] PostHog Cloud, Debug information: [please copy/paste from https://us.posthog.com/settings/project-details#variables]
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit: [please provide]
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: [please provide]

Start by locating the workflow email provider integration and its setup/configuration entry point; the issue names no files or tests. Trace how the built-in provider is configured, then determine the scope for SMTP host, port, credentials, TLS settings, and domain verification. Done means workflows can select and use a custom SMTP provider with those settings.