Sendgrid, like Mailgun, only passes BCC recipients as a parameter in the original JSON payload.
This PR adds code to prepend the recipients from the Sendgrid payload to the raw_email under the X-Original-To header.
References #38738.
Mandrill's Inbound API checks to see if a URL exists before it creates
the webhook. It sends a HEAD request, to which we now return a 200 OK
response to indicate that the route exists.
Now we can generate inbound API calls with ease on Mandrill, without
having to shuffle around tokens in production.
Fixes#37609.
Expose the Mailgun SMTP recipient to Action Mailbox as X-Original-To, like Postfix.
Allows handling BCCed recipients who aren't listed in To/CC fields.
Requiring _just_ the `Aws::SNS::MessageVerifier` does not work as it
references other classes in the AWS SDK that are not in this one class.
Bringing in the entire SNS SDK verifies the authenticity correctly.
