Add SMTP support as an alternative email provider

[ntuan2502]

Problem statement

Currently, Reqcore relies on RESEND_API_KEY for sending emails.

This creates several limitations:

• Requires dependency on a third-party email service (Resend)
• Not suitable for organizations that already have existing SMTP infrastructure
• Adds extra cost and external dependency
• Reduces flexibility for self-hosted deployments
• Some environments (enterprise / on-premise) restrict use of external APIs

For teams running Reqcore in production, especially self-hosted setups, this is a significant limitation.

Proposed solution

Add support for SMTP-based email sending as an alternative to RESEND_API_KEY.

Specifically:

Allow configuration via environment variables such as:

SMTP_HOST=
SMTP_PORT=
SMTP_USER=
SMTP_PASS=
SMTP_FROM=
SMTP_SECURE=true

Application behavior:

• If SMTP config is provided → use SMTP for sending emails
• Otherwise → fallback to existing Resend integration

Suggested implementation:

• Use a standard library (e.g. Nodemailer)
• Abstract email provider behind a service layer
• Support both providers (Resend + SMTP)

Optional improvements:

• Support TLS / STARTTLS
• Support custom sender name
• Add connection test endpoint or health check
• Support multiple email providers via adapter pattern

Alternatives considered

• Continue using only Resend
  → Simple but limits flexibility and self-hosting

• Replace Resend entirely with SMTP
  → Not ideal, removes existing working integration

• Use other API providers (SendGrid, Mailgun, etc.)
  → Still external dependency, same limitation as Resend

Scope and impact

This improvement benefits:

• Self-hosted users → full control over email infrastructure
• Enterprises → easier compliance with internal policies
• Developers → more flexibility in configuration
• DevOps teams → easier integration with existing systems

Success can be measured by:

• Increased adoption in self-hosted environments
• Reduced issues related to email setup
• More flexible deployment configurations

Checklist

• I searched existing issues and did not find a duplicate.
• This request aligns with Reqcore's ownership/transparency/privacy direction.

[JoachimLK]

Thank you for such a well-thought-out feature request with clear problem statements and a solid proposed solution.
SMTP support has been implemented in PR #157 using Nodemailer as the transport layer, alongside the existing Resend integration:

Six new environment variables are supported: SMTP_HOST, SMTP_PORT, SMTP_USER, SMTP_PASS, SMTP_FROM, and SMTP_SECURE.

The email utility now uses an internal unified send helper that transparently routes through SMTP when SMTP_HOST is set, and falls back to Resend otherwise, existing deployments require zero changes.
Auth/credential fields are optional to support open relays.

The .env.example and SELF-HOSTING.md have been updated with full documentation for the new variables.

This gives self-hosted deployments complete control over their email infrastructure. Thank you again for raising this, it's a great addition for the community!