If you no longer have access to the email address and have forgotten the password, the normal password-reset process may not work. I would suggest using the “Send Feedback” option on Family Echo to contact the site administrator directly.
Explain that the account was created with an organization email that has since been discontinued and provide as much information as you can remember, such as your old email address, username, approximate date the tree was created, and the names of a few people in the tree. This may give the administrator enough information to determine whether the account can be recovered.
I would avoid posting any sensitive information publicly on the forum. Similar Family Echo users have suggested contacting the developer through the feedback option for forgotten-account situations.