Bulk email sending fails for Mailgun starter accounts with larger lists

[knowsuchagency]

Users with Mailgun starter accounts (100 emails/day limit) cannot send newsletters to lists larger than 100 subscribers. Ghost currently batches emails in groups of 1000, immediately exceeding daily limits.

Steps to Reproduce

1. Subscribe to Mailgun with a new/starter account (100 emails/day limit)
2. Add 200+ subscribers to Ghost
3. Try to send a bulk email/newsletter
4. Email fails with rate limit errors

Current Behavior

• Ghost sends 1000 emails per batch by default
• No rate limit detection or handling
• Failed sends require manual retry after 24 hours

Expected Behavior
Ghost should support configurable batch sizes based on Mailgun account tier and automatically handle rate limit errors from the API.

Proposed Solution
Add tier-based rate limiting with configuration:

{
  "bulkEmail": {
    "mailgun": {
      "rateLimit": {
        "tier": "starter"
      }
    }
  }
}

This would automatically batch emails appropriately (100 for starter tier) and handle 429/402 responses from Mailgun.