Every wiki group can have a logged mailing list. All mail sent to the group's email address is archived in the mailing list. Unlike the wiki and blog, content in logged mailing lists can't be changed or commented on. However, logged mail can be tagged, and because they each have a URL, you can link to them.
The group email address is groupname-wiki@serverurl.
For example, if your group's name is Aardvark and the URL of your wiki server is www.example.com, the group email address is firstname.lastname@example.org.