The user@domain format will be fully supported in future - I know a lot of people like it, as it is easier for mail users. So even though Virtualmin has to do a lot of behind the scenes trickery to implement it (which causes bugs like the one reported in this thread), it is still worth the effort.

I've had no issue with it. I'm just wondering if this functionality will stick around, or if you guys will go the way of Wietse and deprecate it at some point?

As Jamie mentioned, it'll stick around. There's no risk of us deprecating it. Jamie worked too hard to support it so well! ;-)

The complaint Wietse has with it is actually solved by our two-user workaround. The two users makes some folks uncomfortable, but it's a harmless little workaround and most tools never need to know about it. And, if you use Maildir, it's almost foolproof. Old style mbox mail spools are still a bit problematic, and I'm not sure there's anything we can do to solve that particular problem. Luckily, pretty much everyone agrees on the benefits of Maildir these days--even if arguments over performance remain, it's hard to argue that Maildir doesn't work better for large mailboxes in a virtual hosting environment (benefits range into many disparate areas like backups, folder management, mass deletions for spam, etc.).

Anyway, now I'm just rambling. Don't worry. We support user@domain mailboxes and will forever and ever, amen.