mail sending (SMTP) problem
 

what are the causes for recieving error messages in my mail log, using mailq command,

deferred: resource temporarily unavailable

this is happening for any domains i try to send emails

I think this usually indicates issues in resolving the hostnames. I'd start by checking DNS.

let me show excerpts from the mail log:

[13948]: kAB6ksoi009373: to=<ab@xyz.com>, delay=13:3
9:43, xdelay=00:00:00, mailer=esmtp, pri=1295378, relay=clust.domain.com., dsn=4.0.0,
stat=Deferred: Connection reset by clust.domain.com.

doin a nslookup gives me satisfactory result. what shall be the pausible causes here, if i need to look into my network parameters, which area shall i start with

It looks like it is trying to relay the mail to the 'clust.domain.com' machine. However, that machine is closing the connection. I'm guessing it is a problem with 'clust.domain.com'. Are you using that machine as a spooling server?