这个错误偶尔会发生，我找不到这个问题的根本原因。
请帮助我解决这个问题。
最佳答案
SMTP 状态代码
4.2.1
The Mail transfer service is unavailable because of a transient event. SMTP reply
421 can be caused by many things but generally indicates that the mail server which returns this status code is currently unavailable but may be available later.
For example, the server administrator may have stopped the mail service to troubleshoot a problem, or the mail server is right in the middle of rebooting, or the mail server is currently processing too many incoming messages or incoming requests, etc... Note : “Mail Server” in this case can be any of the mail servers on the message’s route – the sending server (your server), the ISP SMTP server, or the recipient’s mail server.
Clearly, if you repeatedly receive an SMTP status 421 then the problem is no longer of a transient nature and you need to investigate or inform the relevant network administrator, ISP tech support, or the recipient.
SMTP Response 421 can also be received as a result of your message server sending an email where the total number of TO, CC, and BCC users results in a number of simultaneous SMTP connections that is in excess of the number of connections your ISP or SMTP service allows. A typical error message for this situation would be : “421 Too many concurrent SMTP connections from this IP address; please try again later”. Typically, when this happens your server will have sent some of the messages (note that for all servers, each email sent by a user always gets broken down into individual separate emails to each of the recipients in the TO, CC, and BC fields), and will automatically retry a little later to send the remaining messages.
关于java - 使用 java 的电子邮件通知我可能会遇到错误，因为无法连接到 SMTP 主机 : RELAY. MY.COM，端口 : 25, 响应:421，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/24330874/