I you want the easiest way to check this, just go to /home/username/mail/domain.com/email_username and type 'ls -al'
If you see something like:
drwxr-x--- 2 username mail 4096 Mar 16 19:57 ./
drwxr-xr-x 8 username mail 4096 Apr 13 2005 ../
-rw-rw---- 1 username mail 18739207 Mar 16 19:57 inbox
It's mbox, and if you see something like:
drwxr-x--- 14 username mail 4096 Feb 13 16:56 ./
drwxr-xr-x 17 username mail 4096 Jan 4 18:38 ../
drwx------ 2 username mail 28672 May 21 22:19 cur/
-rw-rw---- 1 username mail 12 Nov 15 2005 .mailboxlist
-rw------- 1 username mail 3687 May 21 21:54 maildirsize
drwx------ 2 username mail 4096 May 21 22:19 new/
It's maildir
The point is that mbox keeps all e-mails in one file, maildir keepsa them (I'm sure you can guess) in one folder/direcroty