Linux redhat7.1 smtp problem

I try to setup email server in my Linux redhat7.1 thru KDE,
but unfortunately i can’t get the smtp running.I used the kmail
service provided in KDE.I can send mail using sendmail but when I try to send mail using SMTP it comes out this error
“sending failed”
“cannot open SMTP connection to host myservername.mydomain.com for sending,the message will stay in the outbox for folder and will be resent
Please remove it from there if you do not want the messageto be resent
The following transport was used “SMTP://myservername.mydomain.com:25
I can download mails thru my pop3 account
Pls advise.