From a company I've worked with for seven years now (but is also infamous for REDACTED UPON FURTHER THOUGHT) :
Thank you for attending --------’s callback audition last week. Thank you for your energetic response to the exercises and for all of your proposals. It was a pleasure having a chance to work with you, as usual. It was a hard choice to make amongst such a talented group, but we have determined who will be called back for a second set of callbacks. Although you have not been selected for this particular production, we hope that there will be an opportunity to call you in for future auditions and to work with you on upcoming projects.
Seriously, I could do without these rejection emails.