with respect to/in reference to/regarding/concerning are all commonly used in letters and emails.
I agree that "in response to" is also appropriate, especially when you're dealing with an enquiry.
"this mail" is superfluous. I would probably write;
"I am writing in reference to your enquiry concerning..."
I'm a civil servant. At work, in a formal letter or email, I would write "Thank you for your enquiry about ..." The informal version, which I'd only use in an email, would be "Thanks for your enquiry about".