How to send mail from app in background

I need to send a mail(s) in the background for my app. What is the best way of doing this? I have tried Mailgun, but hav not got it working. Any tips on what the best/easiest way to send mail in background? Here´s the code I was trying for Mailgun:

let parameters = [
                    "from": "my@mail.com",
                      "to": "you@mail.com",
                 "subject": "Test",
                    "text": "Testing"]
     let header : HTTPHeaders = [
             "Authorization": "MY-KEY",
             "Content-Type" : "application/x-www-form-urlencoded"]

     let url = "MY-URL"
     AF.request(url,
                    method: .post,
                parameters: parameters,
                  encoding: URLEncoding.default,
                   headers: header)
             .responseJSON { response in
                 print("Response: \(response)")
}

This throws the following error:

failure(Alamofire.AFError.responseSerializationFailed(reason: Alamofire.AFError.ResponseSerializationFailureReason.jsonSerializationFailed(error: Error Domain=NSCocoaErrorDomain Code=3840 "Invalid value around character 0." UserInfo={NSDebugDescription=Invalid value around character 0.})))