Hi
I want to know if the email address in the mailx exists or not
Eg:
Mailx -s "Subj" hello@ab.com
How do I know if the email address is a valid one???

Sorry, there is no way to do that. If everyone followed the rfc's exactly, the mail would be delivered or returned. That might take days or even weeks. But lots of sites just silently discard undeliverable mail. If your servers are not configured correctly, the mail can arrive at a destination host without a vaild return address. Spammers do this intentionally. (A spammer once sued AOL for returning undeliverable mail as per the rfc's.)

ping will tell you if a site is up or whois will tell you about a valid domain name.

finger username@host.com (which is frequently turned off) will tell you if the username is valid for the domain name.

Like Perderabo says, mailx and sendmail don't know about username at the time email is sent.

Can I get the exit status of finger username@host.com
as in if the username@host.com exists or not?

finger does return a status but if you read my earlier post, it is turned off on a lot of servers. So, the status you get is essentially meaningless.

If you described what your problem is, there may be another solution rather than checking usernames on every email.