The old mailing lists have been migrated from Sourceforge to Google Groups. You only need a Google account to reply via the web interface. If you post via email you can use whatever email you want. In order to cut down on spam, you must join the group before posting. However, anyone and everyone is allowed to join.
scantailor-devel is for developers about the maintenance and expansion Scan Tailor.
scantailor-users is for user support and general questions.
Scan Tailor has a section on the DIYBookscanner forums. There are a lot of Scan Tailor users and fans on the DIYBookscanner site, and they are generally a friendly group.