Our previous web server was Windows based, and it hosted multiple domains. As a result, if a user with name "username" wanted to log in via FTP to "example.com", they'd provide their username as "email@example.com".
We've modified things since then; our web server is now using CentOS 6, and our app no longer relies on FTP for anything. However, there may still be some stragglers out there who haven't updated, and I was curious if it's possible to set up some FTP users to emulate the old behavior, sort of as a little backwards compatibility until we know that no one's relying on it anymore. Trying to create a user with an '@' in their name results in the somewhat predictable:
useradd: invalid user name 'firstname.lastname@example.org'
It's probably not possible to coerce the system to accept a name like that, but I don't know if there's any system for something like those "virtual domains" when users log in that's anything similar to what we were doing on Windows.