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

One of the common category of mail issues are those that result from failed client-server connection. Here we’ll go through the reasons why this error is returned in cPanel servers, how to identify the issues, and how to resolve them.

1. Customer IP blocked in server firewall

The top reason for this error in cPanel servers is the listing of customer’s IP in server firewall. Too many failed logins, simultaneous POP connection requests from office IP, or a website update that triggers mod_security can cause the IP to be blocked.

To find if the IP is listed in the server, just use the command below:

# iptables -L INPUT -v -n | grep "xx.xx.xx.xx"

If it is listed, just unblock it using either of the commands below:

# csf -dr xx.xx.xx.xx
OR
# iptables -D INPUT -s xx.xx.xx.xx -j DROP
# service iptables save

2. Customer’s PC antivirus blocking connection

If the customer’s IP is not listed in the server, look for clues in the support request about a recent update, or a recent installation of antivirus software. Email scanning feature of antivirus suites are known to prevent outgoing mail connection. Disabling the antivirus for a couple of minutes will reveal if the issue is related to that.

3. Customer’s PC firewall settings

Like the antivirus software, firewall is another PC suite that can interfere with mail. If the priority of Outlook is set incorrectly or if the custom SMTP port (eg: 587) is not added in the firewall, this error is returned. Asking the customer to disable the firewall for a couple of minutes can help you identify the issue.

4. Corrupted Outlook profile

In some rare cases, the Outlook profile could be damaged, causing this error. If the above 3 causes are ruled out, re-adding the account into Outlook or repairing the Outlook PST file using “Microsoft PST repair tool” can fix the issue.

Knowing the common causes of an issue will save time in restoring a customer’s mail service. Asking the right questions upfront, and giving the solution options in the first few minutes reduces customer hold time, and improves support efficiency.
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