Here is a solution if you are not receiving automatic email notification of new replies to that thread you are subscribed to.

First, confirm that the following setting in "UserCP" - "Edit Options" are set this way. Some may be optional but start with these.

Login & Privacy
Uncheck: Use Invisible Mode
Uncheck: Allow vCard Download

Messaging & Notification
check: Receive Email from Administrators
Check: Receive Email from Other Members
Check: Receive Friendship Request Email

Default Thread Subscription Mode
Select: Instant email notification

Private Messaging - Optional
Check: Enable Private Messaging
Uncheck: Receive Private Messages only from Contacts and Moderators
Check: Show New Private Message Notification Pop-up

Visitor Messaging
Check: Enable Visitor Messaging
Uncheck: Limit usage of Visitor Messages to Contacts and Moderators

Miscellaneous Options
Message Editor Interface - Select
Enhanced Interface - Full WYSIWYG (What you see is what you get) Editing

You will also have the choice on setting a "Notification Type" in "Additional Options" with replying to a post.

If you are still unable to get email notifications then it's a good chance your ISP (Comcast.net for example) is blocking the Gmail address HDF is using.

If you have another email account setup try it first.
The following are known to work.

Gmail.com
Yahoo.com
MSN.com

If you do not have another account, Gmail is a good choice. The advantage of using Gmail is it can be setup as a POP or IMAP client and email can be accessed through several email applications including MS Outlook and Outlook Express. Oh, and it is free.

The Gmail account can also be setup to forward mail to an another email account automatically. This may be the easiest way to set it up.