我有这个错误，我不能修复，我不知道是什么错了。我被困在这里。
这个错误意味着什么？com.google.apphosting.API.apiProxy$呼叫未找到异常：无法进行API调用邮件。请发送一个既不是原始请求线程也不是ThreadManager创建的线程
这是完整的堆栈跟踪：

org.apache.commons.mail.EmailException: Sending the email to the following server failed : smtp.gmail.com:587
    at org.apache.commons.mail.Email.sendMimeMessage(Email.java:1242)
    at org.apache.commons.mail.Email.send(Email.java:1267)
    at com.wealthsystems.sim3g.modulo.email.impl.EnviaEmailsThread.process(EnviaEmailsThread.java:123)
    at com.wealthsystems.dao.hibernate.api.service.WsThreadService.run(WsThreadService.java:55)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
    at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:304)
    at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:178)
    at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    at java.lang.Thread.run(Thread.java:745)
Caused by: com.google.apphosting.api.ApiProxy$CallNotFoundException: Can't make API call mail.Send in a thread that is neither the original request thread nor a thread created by ThreadManager
    at com.google.apphosting.api.ApiProxy$CallNotFoundException.foreignThread(ApiProxy.java:800)
    at com.google.apphosting.api.ApiProxy.makeSyncCall(ApiProxy.java:112)
    at com.google.apphosting.api.ApiProxy.makeSyncCall(ApiProxy.java:65)
    at com.google.appengine.api.mail.MailServiceImpl.doSend(MailServiceImpl.java:101)
    at com.google.appengine.api.mail.MailServiceImpl.send(MailServiceImpl.java:34)
    at com.google.appengine.api.mail.stdimpl.GMTransport.sendMessage(GMTransport.java:223)
    at javax.mail.Transport.send(Transport.java:95)
    at javax.mail.Transport.send(Transport.java:48)
    at org.apache.commons.mail.Email.sendMimeMessage(Email.java:1232)
    ... 10 more

这是我的代码，生成此错误：

public static void sendMailApache(String protocol, String host, String port, String userName, String password, 
           String subject, byte[] content, String para, String cc, String co) {
       try {
           final Email email = new SimpleEmail();
           email.setHostName(host);
           email.setSmtpPort(Integer.parseInt(port));
           email.setTLS(true);
           email.setSSL(false);
           email.setAuthenticator(new DefaultAuthenticator(userName, password));
           email.setFrom(userName);
           email.setSubject(subject);
           email.setCharset(org.apache.commons.mail.Email.ISO_8859_1);

           for (String str: para.split(";")) {
               email.addTo(str);
           }
           for (String str: cc.split(";")) {
               email.addCc(str);
           }
           for (String str: co.split(";")) {
               email.addBcc(str);
           }
           email.setMsg(content.toString());
           email.send();
       } catch (EmailException e) {
           throw new RuntimeException(e);
       }
    }

这个错误意味着什么？我做错了什么？