I was having trouble setting up a mail user with imap2.us.opalstack.com as the server. I tried logging in to reach them via webmail and through Thunderbird, but both gave me error messages when I attempted to log in to them. I tried sending an e-mail to the address I setup as the source to this destination mail user, but it was returned with a "Host or domain name not found." error.
I then tried imap1.us.opalstack.com and everything worked perfectly. I went back and created another mail user with imap2.us.opalstack.com and ran into the same issue with logging in via webmail. It looks like there might be something wrong with this server. Or, since it's not listed in the documentation, maybe the bug is that it shouldn't be an option right now?