HELP! Columbia Letter of Recommendation

I’m applying to Columbia Early Decision, and about a month ago, I asked a few of my teachers to write letters of recommendation for me. I’ve found information on Columbia’s undergrad website about how to submit these letters, but one of them recently contacted me saying that he hasn’t written my letter yet because he doesn’t know what sort of information they want in these letters. I told him that their website doesn’t offer much information, but he insists on having better information before he begins writing. I’ve also contacted undergrad-ask@columbia.edu, but I assume that it will take a few days before I get a response from them. Does anyone know what Columbia is looking for in a letter of rec? I believe that there is an email address that would allow me to contact a Columbia student directly, but I forgot the specific address.