I'm not able to send mails from my server using MAILX command,have found that some of them,sendmail.cf has no entry for DS.Is there any other things which can disable mail functionality in server?
The absent DS entry might not matter.
The sendmail daemon needs to be running, twice (at least). Since version 8.12 of sendmail, it uses a split queue model: ps -ef should show one sendmail daemon process owned by smmsp which takes the mail you sent and hands it over to the one owned by root for delivery.
There are many ways to configure mail on Solaris. Often both inbound and outbound mail processing will be centralised on particular servers (for ease of spam filtering, virus checking, alias expansion, and so on, and so that users have only one mailbox each). You should speak to your system administrator about how mail is set up on your site.
There is a very clear description of the three most common configurations in this short article at Sun's BigAdmin site. You probably want the last one: outgoing only.