Mailgun Provider needs option for EU server (domain is different)

[HexaCubist]

Provider type

Email

Environment

  System:
    OS: macOS 15.4.1
    CPU: (8) arm64 Apple M1
    Memory: 108.25 MB / 16.00 GB
    Shell: 4.0.1 - /opt/homebrew/bin/fish
  Binaries:
    Node: 20.15.1 - /usr/local/bin/node
    Yarn: 1.22.22 - /usr/local/bin/yarn
    npm: 10.7.0 - /usr/local/bin/npm
    bun: 1.2.4 - ~/.bun/bin/bun
  Browsers:
    Chrome: 135.0.7049.115
    Safari: 18.4
  npmPackages:
    @auth/drizzle-adapter: ^1.9.0 => 1.9.0 
    @auth/sveltekit: ^1.9.1 => 1.9.1

Reproduction URL

https://github.com/HexaCubist/next-auth-mailgun-eu

Describe the issue

Mailgun has two regions to send from since 2018: https://www.mailgun.com/about/regions/

API keys can be created for one region only, and the US region is hard-coded into the provider:

• US Domain: api.mailgun.net
• EU Domain: api.eu.mailgun.net

mailgun.ts:

...
      const res = await fetch(`https://api.mailgun.net/v3/${domain}/messages`, {
...

How to reproduce

1. Clone repo
2. Provide EU API keys for Mailgun
3. Error:

[auth][error] Error: Mailgun error: Forbidden

Expected behavior

Mailgun provider should have configurable option to select region (EU or US), defaulting to the US.

[HexaCubist]

Related: #11923