Apologies - I thought that the documentation for this had been finished and added to the Moodle docs, but I'm unable to find it. I'll ensure that they're added as soon as possible.
For the moment I'll quickly describe what you need to set and then I'll turn these notes into proper documentation.
For the Reply to post feature to work, you need a dedicated IMAP account. This account should not be used by anyone else. It should not be a personal account, or one which is automatically checked by another client (e.g. Outlook). When the task runs to fetch messages from the IAP account, it makes changes to messages in the account, and the account structure (it reads all of your mail, flags it, and moves some of it to other folders).
You will need the following information to hand:
Inbound address (i.e. the bit before the @ sign when you send this account mail. In the address email@example.com, that would be moodle
(i.e. the bit after the @ sign when you send this account mail. In the address firstname.lastname@example.org, that would be example.com
).IMAP server (e.g. imap.gmail.com);IMAP username (the one you would use to log in to check mail manually); and IMAP password (the one you would use to log in to check mail manually).
You may additionally need to know what security settings your IMAP server uses but either of Automatically detected SSL, or Off are the best settings to use.
After logging in as an administrator, navigate to Site administration -> Server -> Incoming mail configuration -> Mail settings and enter the above details. You will also need to tick the "Enable incoming mail processing" setting at the top of the page.
Now open the "Message handlers" page from the same part of the administration tree. You can select which features you enable and configure any default options for them. For "Reply to post", you need to edit the "Reply to forum posts" handler. Here is an example of a filled-in settings page.
Message validity for the forum defaults to one week, but you can configure it. After this time, the handler will reject the message, the argument being that, with a week old message, you may want the user to view the message on the forum to ensure that there haven't been subsequent replies that they have missed, etc. and which may be relevant.
It's up to you whether you require validation of the sender's e-mail address. This ties in with one of the other handlers (Invalid recipient handler). Essentially this means that if a user does not use the e-mail address in the user's profile, then Moodle will send an e-mail to their profile address asking them to confirm the message. If the Invalid recipient handler is disabled, then the message will be rejected. If you don't wish to have this additional level of checking, theny you can disable the check entirely for the reply to post handler.
The only other part of the setup is to ensure that you have cron configured to run regularly.
Hope that this helps,