Mentioning a Professor and his comment in a mail without his permission Can I mention someone's(academic) name and comment which I got from a mail, to another academic (to increase the credibility of my work) without informing the first academic that I am going to use his name and comment?

For example, I sent my work through an e-mail to Professor X( Ph.D ). He wrote me back briefly that   -

"the analytical results are correct
but they are not practicable. You should work on approximation algorithms for .... ...."

I think the part "analytical results are correct" adds some credibility to my work and I would like to mention  Prof X and his comment(whole comment)  when I write my next mail to someone else. Since, academics have issue with mail from strangers(tagged as 'crackpot' or 'crank'), this comment might interest them.

But do I have to ask permission from Prof. X to mention his name and comment to another academic ?
You should ask permission from Prof. X.

Quoting a statement like "the analytical results are correct" implies endorsement to some degree. 

You should be very careful not to say anything that could imply endorsement without permission of the "endorser." 

Note that if he does give permission, you still have to be careful not to quote his statement out of context. For example, you should not say 


  Prof. X said my analytical results are correct.


instead, you should explain what he really said, more like


  I showed this work to Prof. X. He said that the analytical results seem correct, but not practical, and he suggested that I look into approximation algorithms.