This is from the www.dimeadozen.org homepage:

" Attention users of client Mainline (BitTorrent) v4.22 and above:

Mainline (BitTorrent) v4.22 and above is anything else but a reliable BitTorrent client. It doesn't adhere to Bram Cohen's own protocol specs of v1.0 anymore, so we had to ban it from our trackers today (April 26, 2007). Sorry for that. See our FAQ for recommended clients."

I would switch to something like BitTornado 3.18, which has encryption or Azeurus, which also has encryption. Much better than the Official Bittorrent client. I use BitTornado 3.18 and am very happy with it.