1. review smtp logs in C:\Windows\System32\LogFiles\SMTPSVC1,
2. find 421 error
2014-12-08 02:02:40 212.118.102.234 OutboundConnectionResponse SMTPSVC1 SMTP1 - 25 - - 421+Service+not+available,+closing+transmission+channel 0 0 55 0 109 SMTP - - - -
2014-12-08 02:02:40 212.118.102.234 OutboundConnectionResponse SMTPSVC1 SMTP1 - 25 - - 421+Service+not+available,+closing+transmission+channel 0 0 55 0 109 SMTP - - - -
2014-12-08 02:02:40 212.118.102.234 OutboundConnectionResponse SMTPSVC1 SMTP1 - 25 - - 421+Service+not+available,+closing+transmission+channel 0 0 55 0 109 SMTP - - - -
2014-12-08 02:02:40 212.118.102.234 OutboundConnectionResponse SMTPSVC1 SMTP1 - 25 - - 421+Service+not+available,+closing+transmission+channel 0 0 55 0 94 SMTP - - - -
2014-12-08 02:02:40 212.118.102.234 OutboundConnectionResponse SMTPSVC1 SMTP1 - 25 - - 421+Service+not+available,+closing+transmission+channel 0 0 55 0 94 SMTP - - - -
2014-12-08 02:02:40 212.118.102.234 OutboundConnectionResponse SMTPSVC1 SMTP1 - 25 - - 421+Service+not+available,+closing+transmission+channel 0 0 55 0 94 SMTP - - - -
2014-12-08 02:02:40 212.118.102.234 OutboundConnectionResponse SMTPSVC1 SMTP1 - 25 - - 421+Service+not+available,+closing+transmission+channel 0 0 55 0 94 SMTP - - - -
3. check this ip
nslookup 212.118.102.234
Server: server1
Address: 192.130.74.51
Name: mail.zahid.com
Address: 212.118.102.234
4. telnet port 25 for this server on smtp, return 421 error
421 Service not available, closing transmission channel
Connection to host lost.
5. telnet port 25 for this server on other server, return 220, connection established.
Xshell:\> telnet 212.118.102.234 25
Connecting to 212.118.102.234:25...
Connection established.
To escape to local shell, press ‘Ctrl+Alt+]‘.
220 *************************************************
6. So 212.118.102.234 block our SMTP external IP address, and many emails are in queue to ready to delivery again.......
refer: http://simplydotnet.wordpress.com/2010/08/11/outgoing-email-stuck-in-queue-folder-iis-smtp-server/
原文：http://www.cnblogs.com/oskb/p/4150572.html