From last 2 days, my few users are getting below message while sending mails :

"Your server has unexpectedly terminated the connection. Possible causes for this include server problems, network problems, or a long period of inactivity. Subject 'TM', Account: 'abc', Server: 'mail.example.in', Protocol: SMTP, Port: 25, Secure(SSL): No, Socket Error: 10053, Error Number: 0x800CCC0F"

They are using Outlook express with pop3 setting.

My Server configuration is :

Enable POP3/IMAP service = Yes
Enable SSL for POP3/IMAP = no
Enable clear test login for POP3/IMAP = Yes

Right now 4 users got affected and getting same message while sending. they are able to recevie the mails without any issue.
This morning one of the user was able to send mail but while sending attchment he got same error. and now again he is not able to send mail without attchement. This is very strange.

I google this error and got very helpful link from microsoft :

Troubleshooting error messages that you receive when you try to send and receive e-mail in Outlook and Outlook Express

Server Error: 451, Socket Error: 10053, Error Number: 0x800CCC0F - Windows XP

As per Microsoft support below are the reasons to get such error while doing send / receive. I have performed all possible steps as mentioned below :-

1. You are not connected to the Internet or a network, or your mail server is temporarily unavailable ---> I am able to telnet to mail.example.in from user's desktop on port 25 and 110
2. Your account settings are incorrect - I have personally checked user's configuration and server account settings. Both are perfect.
3. The configuration of your AV software is incorrect --> disable the antivirus
4. The configuration of your personal firewall software is incorrect --> I have disabled windows firewall.
5. An e-mail item on your POP3 server is damaged. --> The same account is working fine in internet lab without any issue. Means there is no problem with user's mailbox account
6. Your user profile in Outlook is damaged. --> Need to check
7 . Outlook Express was removed from the computer or the installation is damaged. --> Need to check

Now i asked my other support team to reinstall outlook express on one of the user for testing purpose..but now i again got one more call regarding same error message ...so now I am feeling that do i need to perform any steps from server end ??? so just wanted to confirm with you .....

Have anyone one ever face such issue ??

Please suggest and help...