在Linux下使用JavaMail发送邮件报错：javax.mail.MessagingException: Unknown SMTP host:

使用JavaMail发送邮件， 在Windows上测试都是OK的，但发布Linux上，测试后报Unknown SMTP host异常。

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异常信息如下： 
javax.mail.MessagingException: Unknown SMTP host: smtp.126.com; 
  nested exception is: 
java.net.UnknownHostException: smtp.126.com 
at com.sun.mail.smtp.SMTPTransport.openServer(SMTPTransport.java:1704) 
at com.sun.mail.smtp.SMTPTransport.protocolConnect(SMTPTransport.java:525) 
at javax.mail.Service.connect(Service.java:297) 
at javax.mail.Service.connect(Service.java:156) 
at javax.mail.Service.connect(Service.java:105) 
at javax.mail.Transport.send0(Transport.java:168) 
at javax.mail.Transport.send(Transport.java:98) 
at com.utvchina.cas.util.MailSender.sendHtmlMail(MailSender.java:107) 
at com.utvchina.cas.util.MailSender.send(MailSender.java:133) 
at com.utvchina.cas.util.VerificationInfo.send(VerificationInfo.java:64) 
at com.utvchina.cas.action.RegisterAction.save(RegisterAction.java:124) 
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) 
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) 
at java.lang.reflect.Method.invoke(Method.java:597) 
at com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:441) 
at com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:280) 
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:243) 
at com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:165) 
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87) 
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) 
at com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:252) 
at org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68) 
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87) 
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) 
at com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:122) 
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) 
at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:195) 
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87) 
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) 
at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:195) 
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87) 
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) 
at com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:179) 
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) 
at org.apache.struts2.interceptor.MultiselectInterceptor.intercept(MultiselectInterceptor.java:75) 
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) 
at org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:94) 
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) 
at org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:235) 
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) 
at com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:89) 
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) 
at com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:130) 
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) 
at org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:267) 
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) 
at com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:126) 
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) 
at com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:138) 
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87) 
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) 
at com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:165) 
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) 
at org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164) 
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) 
at com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:179) 
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) 
at com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:176) 
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) 
at org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:52) 
at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:488) 
at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77) 
at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:91) 
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157) 
at com.github.inspektr.common.web.ClientInfoThreadLocalFilter.doFilter(ClientInfoThreadLocalFilter.java:46) 
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157) 
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388) 
at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216) 
at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182) 
at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765) 
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:440) 
at org.mortbay.jetty.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:199) 
at org.mortbay.jetty.handler.HandlerCollection.handle(HandlerCollection.java:114) 
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152) 
at org.mortbay.jetty.Server.handle(Server.java:326) 
at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542) 
at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:943) 
at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:756) 
at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:218) 
at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404) 
at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:410) 
at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582) 
Caused by: java.net.UnknownHostException: smtp.126.com 
at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:177) 
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:366) 
at java.net.Socket.connect(Socket.java:529) 
at java.net.Socket.connect(Socket.java:478) 
at com.sun.mail.util.SocketFetcher.createSocket(SocketFetcher.java:284) 
at com.sun.mail.util.SocketFetcher.getSocket(SocketFetcher.java:227) 
at com.sun.mail.smtp.SMTPTransport.openServer(SMTPTransport.java:1672) 
... 82 more

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解决方法：
这个问题主要是软防火墙(iptables)中邮件发送端口未对外开放。 

邮件端口为25 

修改iptables即可，步骤如下： 

1、修改/etc/sysconfig/iptables文件，增加如下一行： 

　　-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 25 -j ACCEPT 
　 

2、重启防火墙，这里有两种方式重启防火墙 


　　a) 重启后生效 
　　开启： chkconfig iptables on 
　　关闭： chkconfig iptables off 



　　b) 即时生效，重启后失效 
　　开启： service iptables start 
　　关闭： service iptables stop
还有一个可能的原因就是：DNS未配置或配置不成功(可ping测试一下)