Trap email of bounced address and add to db if server is running Postfix? 
We run a debain server running postfix.
We want to prevent mailing to non existent email ids on
yahoo/hotmail/aol etc. The response we get from the email servers when
there is no mailbox by that name is received by postfix and is logged.
What we need is apart from logging a way to trap the email address
then and there and quickly store it to mysql table so that henceforth
we do not try to mail to that address ever.

I have no idea how to do this, but do not understand why you would
want to. What happens if the email address subsequently becomes valid?

I also get the idea and understand its point, that would reduce
irrelevant traffic and reduce bounced emails. However a currently not
working email may get to work again, for example if it was only
temporarily full, so you may not want to "blacklist" your emails once
for all.
Instead of that idea I'd suggest setting up a system that verifies
whether an email address exists, and if the mailserver on which it
resides will accept mail for it, after what you can fire up your
emailing. This can be done in several ways, I'll elaborate if you are
interested.