Well, it took a ton of time and research, but I finally have a working
Unread virtual mailbox on Dovecot 2.x on Centos 6.0. I have no idea if
it's optimal or correct, but it works perfectly and doesn't raise any
errors or warnings.

Here's what I ended up with:

dovecot.conf: Add the following:

mail_plugins = $mail_plugins virtual namespace { type = private prefix = virtual. separator = . location = virtual:~/Maildir/virtual }

10-mail.conf: Add the following: # default namespace namespace { separator = . prefix = inbox = yes }

20-imap.conf: Add the following: protocol imap { mail_plugins = $mail_plugins virtual }

15-lda.conf: Add the following: protocol lda { mail_plugins = $mail_plugins virtual }

Restart dovecot. Reload your mail client.

Dovecot will create ~/Maildir/virtual

cd ~/Maildir/virtual mkdir Unread

Use your favorite editor and create a file named "dovecot-virtual" contaiing:

# ~/Maildir/virtual/unseen/dovecot-virtual INBOX INBOX.* -INBOX.Trash unseen

Save the file and restart your mail client.

Go to "Subscriptions" in your mail client and subscribe to the Unseen folder.

You should now have a virtual folder that contains all your unread
mail, except for the contents of the Trash folder.

Enjoy!

-- Terry Carmen CNY Support, LLC https://www.cnysupport.com