Getting Error while sending mail form outlook

The connection to the server has failed. 
Account: 'customer-domain.com', 
Server: '174.36.188.24', 
Protocol: SMTP, 
Port: 25, 
Secure(SSL): No, 
Socket Error: 10060, Error Number: 0x800CCC0E

This is because the customer not able to connect to port 25 of mail server.

Reasons

• Server May be Down
• Mail Server May be Down
• Firewall Blocking Port 25

To Verify SMTP server working, run

telnet domain.com 25

If connected SMTP server is working.

Since windows firewall do not Block IP by itself, mostly, it can be server down or customer internet problem.