Well, you get your name on the list and can then have things emailed to you. The old messages are listed by title and you have to "page down" from the beginning of time (I've not explored them all yet).

If you know the thread title you can search here http://www-ssrl.slac.stanford.edu/htbin/hypermailnh/..ECDMAILFILE/ECD/

or at http://www-ssrl.slac.stanford.edu/ecd_archive/ you can choose to see the messages for any one month, but they take forever to download and you keep getting the message that is being replied to, ad infinitum, and all the computor code as well.

Mudcatis easier to find things on, even if most of the things on it are less relevant to me (not to mention utter nonsense).
The ECD site only allows relevant posts, and I get about a dozen emails a day from them (or you can choose to have a days worth at a time)