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How long ago did you e-mail them back? If within the last couple of days, I would wait. If beyond that, have you sent a DV at all? While it's highly likely it's older than the 30 days listed in the FDCPA, sometimes a DV will get the ball rolling.
You're pretty lucky to even hear back from them.
I tried snail mail, email and calling. I got nothing for an entire year before they transferred the debt to the law offices of Daniel N. Gordon, which are a bunch of crooks that threatened me with a judgement on a different debt. I literally called over 100 times and left at least 50 messages, 200 emails, and about 10 letters or so with 0 responses from them. They're still reporting on my tradeline as well, but hopefully after I PIF at the end of the week, I won't have to worry about it.