I have heard that to challenge with experian is only online now. I am currently challenging things with them but they limit you to 120 characters online. This does not give you a whole lot to explain.
They aren't going to give a rat's ass about your explanation anyway so using more than 120 characters to make it is a waste of effort. That said, I've disputed with them by mail and online and gotten similar results either way: sometimes they correct it, sometimes they don't.
Its the NCAC address that Apex posted above. Full explanations are more so a CYA , in case one ever feels the need to sue a CRA for erroneous reporting.
You could do both; use the 120 characters online and let it be known that a follow up is coming via e-mail or postal mail. If they care about the details, they'll look out for it!
Using both risks getting one or the other rejected for "previously investigated", especially with Experian. Do one or the other - not both.
