I'm using "[DOMAIN]_" as "user prefix" in ISPConfig. I've been having troubles when a client register a very log domain name, because ISPConfig limits the entry field lenght in then "create user" page. I think there's no such limit in linux's usernames, I can manually create longer names than those allowed at ispconfig panel. I think it's a bug in ISPConfig. Meanwhile, I'm thinking in reverting prefix to default. Is there any problem in having some users with one prefix and some other with another in the same domain? Thank you very much!