I had to deal with this a while ago and I found out (sorry, but I don't have a reference handy) that underscores are prohibited in the second level domain (exam

I had to deal with this a while ago and I found out (sorry, but I don't have a reference handy) that underscores are prohibited in the second level domain (example.com), but are ok in any further levels (šķaudiens.example.com). Basically subdomain names can be anything that your DNS server allows them to be, just as email addresses can be whatever your SMTP server will handle.