Function mailgun_v3::email::send_with_request_builder

pub fn send_with_request_builder(
    request_builder: RequestBuilder, 
    creds: &Credentials, 
    sender: &EmailAddress, 
    msg: Message
) -> MailgunResult<SendResponse>

Same as send_email but with an externally managed request builder. Use this in case you want to send the mails to a custom API endpoint, e.g. for testing.