What problem would you like to solve? Please describe:
Currently, email-based alert creation in GoAlert is only supported via Mailgun webhook. This feature request aims at providing a cost-effective alternative by allowing GoAlert to work as a basic SMTP server that only receives emails.

Describe the solution you'd like:
I propose an SMTP server configuration for GoAlert that enables receiving of mail. This will require admins to configure an MX record for their GoAlert domain. This would function as any other API endpoint, but with a separate port and unique syntax/protocol.

This solution would also mean that we have multiple potential providers of a single service (Email integration keys) for the first time. Multiple providers would need to be managed, either by only allowing one to be enabled at a time, setting a preferred provider, or requiring domains to be identical.

Describe alternatives you've considered:
The current alternative is continuing to use Mailgun webhooks for all email-based alerts. This option carries potential costs and fewer customization options.

Additional context:
Additional considerations include ensuring that all documentation is updated accordingly, and that the "preferred" provider's domain is used in any in-app links. Potential forward compatibility issues might arise, so it may be beneficial to support both methods of alert creation for migrating from one to the other.