Sendgrid API calls raising HTTP 400 Bad Requests

[sentry]

Sentry Issue: FLAGSMITH-API-4XZ

BadRequestsError: HTTP Error 400: Bad Request
(3 additional frame(s) were not displayed)
...
  File "organisations/tasks.py", line 144, in handle_api_usage_notifications
    handle_api_usage_notification_for_organisation(organisation)
  File "organisations/task_helpers.py", line 182, in handle_api_usage_notification_for_organisation
    _send_api_usage_notification(organisation, matched_threshold)
  File "organisations/task_helpers.py", line 86, in _send_api_usage_notification
    send_mail(

Error processing api usage for organisation REDACTED