Mail server letter back: 421 Service not available

Recently, a friend of the company's leader to send mail abroad, received a letter of withdrawal, <--421 Service not available, closing transmission channel. The following is the message server's unsubscribe content.

Or to briefly analyze the content of the letter back. From the content of the letter of withdrawal is a. CN's domain name reply. com's overseas domain name of a mail, through DNS resolution to each other's mail server is 63 IP address, we are 98 IP address, is through 54771 this port in connection to each other's 25 ports, try the SMTP protocol to start. Then the other server feedback 421 error messages, the other server directly shut down the SMTP connection. In our server telnet to each other's 25 port, the same error message appears. Telnet foreign server 25 port is normal on the client. Ping our mail server domain name, the feedback back IP is 00, as shown in the following figure:

See more highlights of this column: http://www.bianceng.cnhttp://www.bianceng.cn/Servers/Mail/

Based on the message, our server uses an IP address ending with 98 to establish an SMTP connection with the other's server. Immediately get in touch with friends, originally their server has 4 public network IP address, 98 end is the preferred IP address, 00 End is their server's alternate IP address. This is clear, because all are public network IP address, can go out, so the mail server used the preferred IP address out.

Temporary solution: The preferred IP replaced by the end of the 00IP address, the message immediately went out.

The Final Solution: 1, enable 4 network cards, the allocation of different IP address, and then the MD to set the domain name and IP binding; 2, enable a network card, four domain name resolution to the same IP.

This article is from the mail server and network Management notes blog, please be sure to keep this source http://19281928.blog.51cto.com/2327194/554641