Question : DNS delegation

I have DNS server under ISP1 & I have registered multiple domain names under this ISP.
Now I have 2nd ISP. I am planning to shift my SMTP traffic to the 2nd ISP. For that I will be making the MX record change to my domains with the ISP 2 IP address.

But i come to know that if i do it there will be problem of reverse lookup & delegation needs to be configured with the ISP 2.

I would like to know what exaclty will happen if i change only the MX record with ISP 2 IP.
 Why should i configure delegation with the ISP 2. What is exactly DNS delegtation

Answer : DNS delegation

That looks good. Just don't forget to terminate names with "." or the zone name will be appended :)

Chris