I contacted my professor to ask for a recommendation letter and I knew from him that he left my institution and asked me to use another e-mail address for him in the recommendation which is a Gmail. Will this affect the credibility of his recommendation in my application?
Is it a good idea to include my professor who left academia as a reference in my PhD application?
3Why did he leave?– Azor Ahai -him-Sep 30, 2018 at 23:06
I don't know but I think for personal reasons. I didn't really have the chance to ask him about the reason.– M. Z.Oct 1, 2018 at 5:26
It seems unlikely that it would have any effect. I assume that he knows how to write a letter and explain things appropriately. The fact that he isn't an academic any more is less important than the relationship you once had and what he can still say about your prospects.
Of course, it is best if he has left a trail of accomplishments behind in his own work that will reflect on you. But he will likely know to point to that in his letter.
Don't depend too much on a single letter, however. You will want several if the institution will accept them.
People also retire, of course, and sometimes get asked for letters.
It is better to include , for honesty and reputation.