In order to receive mail via Kmail, I have create an account in the network>receiving section of settings.

But I got confused about how to set the Host.

I have the email address zjzjzj@21cn.com
when fill the Host with www.21cn.com or anythingelse.

Check mail always produces the error cannot connect to host.

My question is what is that host mean?
Please help.

It means the same as anywhere else, the machine you collect you mail from. Probably mail.21cn.com or pop3.21cn.com

Of course, if 21cn.com is a webmail only account then you can't use kmail with it.

Actually your pop3 server is pop.21cn.com and the smtp for is smtp.21cn.com - you can normally find out what the mail servers are for a domain name by typing "dig mx" (sans quotes) followed by the domain name (here 21cn.com). Or you can try doing nslookup instead of dig but dig is better.

Hi, thanks for your help.

I found that my old email adress don't support KMail after I got a new email account use the HOST: pop3.163.com

Kmail can connect pop3.163.com and download message successfully.

Another question:

I send email via default sender. Is that sufficient?

Whatever works is okay.

Kmail is just a client application to communicate with the mail servers. It shouldn't make a difference if you use Kmail, Mozilla, Evolution or any other program to do it. It's just a nice easy way to talk to the servers. It like going to the shops. How you get there doesn't really matter by car, bus, or teletransportation. I'm sure kmail will work with your old address. It's just an address to your mailbox - nothing more! You just have to make sure that the settings for the incoming and outgoing mail is what your email service provider has set up. You should ask them if there some sort of authentication required or if the servers are listening on ports other than 110 for incoming mail (pop3) and 25 for outgoing mail(smtp).