Occasionally Odin Plesk customers report inability to send or receive emails reporting the following Outlook errors:

The host 'mail.domain.com' could not be found. Please verify that you have entered the server name correctly. Account: 'username', Server: 'mail.domain.com', Protocol: SMTP, Port: 25, Secure(SSL): No, Socket Error: 11001, Error Number: 0x800CCC0D.

The host 'mail.domain.com' could not be found. Please verify that you have entered the server name correctly. Account: 'username', Server: 'mail.domain.com', Protocol: POP3, Port: 110, Secure (SSL): No, Socket Error: 11004, Error Number: 0x800CCC0D.

The errors may have the following variations:

No connection could be made because the target machine actively refused it.
OR
The host ‘mail.domain.com’ could not be found.

What is this error, and how to fix it?

This error essentially means Outlook is unable to connect to the POP/IMAP or SMTP server. We’ve identified 4 reasons why this could happen:

1. PC’s anti-virus or firewall blocking outgoing connection

One of the top reasons for Error 0x800CCC0D is the server listing customer’s IP in the server firewall. Fail2Ban could list the IP due to too many incorrect logins. To check if an IP is listed in the server use the command below:

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

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

# iptables -D INPUT fail2ban-XXX -s xx.xx.xx.xx -j DROP
# service iptables save

2. Customer IP blocked in server firewall

If the IP is not listed in the server, look for clues in the customer’s report about recent changes in their PC. Anti-virus rules or PC’s Firewall settings could be blocking outgoing connections to non-standard SMTP ports, or could be choking the connection with outgoing mail scanning settings. Disabling the anti-virus and firewall for a minute, and re-connecting can reveal if this is the cause.

3. Corporate firewall or VPN blocking mail connections

Some customers especially those who travel a lot could be using VPN or are in some way would be behind their corporate firewall. Security settings in corporate firewalls are known to deny access to non-standard SMTP ports.

4. Corrupted Outlook profile

If none of the above applies, it is possible that the Outlook profile is damaged somehow. Deleting an re-creating the profile is the fastest way to resolve this error. However, if it cannot be done, “Microsoft PST repair tool” can be used to fix the profile.