Exim is a mail transfer agent used on Unix-like operating systems. It is freely available under the GNU GPL and it aims to be a general and flexible mailer with extensive facilities for checking incoming e-mail.
We do not use Qmail.
Maildir is a widely-used format for storing e-mail that does not require application-level file locking to maintain message integrity as messages are added, moved and deleted. Each message is kept in a separate file with a unique name. All changes are made using atomic filesystem operations so that the filesystem handles file locking concurrency issues. A Maildir is a directory (often named Mail) with three subdirectories named tmp, new, and cur.
We do not use mbox.
If you have mbox, you can create the email accounts in cPanel and then email support@hostgator.com and ask us to convert them to maildir.