I think I understand the DNS part . . . haven't set it up yet, but I'm working on it.
I still can't get the mailuser thing to work. I set up the email "email@example.com," which added the prefix "web2_" to it. So, the username is now "web2_info." I gave it administrator and shell access priviledges and I made sure "mailuser" was selected on the site. I sent a test email to "firstname.lastname@example.org" and "email@example.com," but I still can't login to archsupply.com:81/mailuser. I still get this error, "Error: Password error: Maildir: No such file or directory." Does it take some time to configure?