Could anybody please help me to understand,
1. what is DNS flip?
2. what happens when MX record does not exist for domain ? Who takes initiative and returns error to mailer ?
3. and how mailertable works in sendmail?

Sorry if i have put too many questions in single thread.

- That's something like "fail-over", also, as far as I know, it's a commercial tool maintainted by a hosting company, which actually does something like a "cache refresh of DNS records".
The MXLOOKUP call will fail, and bounce-back will be returned to the sender.
3. For mailertable see here.