why do some discussions and even some of the digest fall in the spam folder?

Same with me. I use GMail and I guess its because myLot sends lot of them!

Yeah, my email have the same problem too. May be because email sender using different ip address when sending those email, so email provider think that it is a scam or indicate it as a spam. One thing for you to remember, if you don't want to get that problem again, try add "non-spam" email address on your address list, that will make things easier, because email provider will always allow it, it's like a pass-card for the email address. I usually do that to every contact I have, not only that but it makes you easier for composing email.