Howdy all,
We're always tweaking the default installation to make Virtualmin easier to use, faster to get up-and-running, and to make it Just Work for more people more of the time without any tweaking.
To that end, I'm making a pretty big user-visible change in the next release of virtualmin-config (this is the package that handles all of the postinstall configuration, but can also be called at any time using the virtualmin config-system command, though most users never need to call it manually).
Right now, a fresh install of Virtualmin uses a mailbox user name format of user.domain. e.g. my user on virtualmin.com would be joe.virtualmin, and that would be the user I would give in my mail client (and SSH or FTP client, if connecting that way). Relatively recent changes in Thunderbird have made it rather difficult to configure an account with a username in this format (you have to kinda trick it by entering something like joe.virtualmin@virtualmin.com, which is stupid and annoying and confusing for non-technical users who don't know anything about the backend). So, in the next version of virtualmin-config the default will be user@domain.tld.
This has the annoying quirk that you have to have duplicate users in the passwd/shadow files, because some software doesn't work with @ in the username. But, for the end user, it is clearly superior. So, that's what we're doing. With this change, now only the administrator needs to know WTF is going on with the usernames. Users can just do what feels right (fill in user@domain.tld in mail clients) and have it probably Just Work.
You can, of course, switch to any other username format in the Server Templates (determining the format when the domain is appended to a username) and in System Configuration (determining whether the domain is appended at all or not; in the case of a server with only a small number of domains you may want to just use, e.g. joe instead of joe@virtualmin.com or joe.virtualmin).
Holler if you hate this idea, but I think my mind is made up. Changing it is probably one of our most common requests these days, so I kinda suspect this is more in line with what the average user wants, anyway.