1. Go to Dashboard > System & Settings > Mail > Mail Importers.
2. You should see a listing for Discussion Post Reply. Click Edit.
3. Enter information for an email address to use as the reply email address. This is a mailbox that is completely dedicated to this task – something like "discussions-reply@yourdomain.com." You'll have to set this email address up yourself. Enter the information to connect to the email address account (POP/IMAP, etc...)
4. Make sure that the mail importer is "Enabled."
5. Now, head over to Dashboard > System & Settings > Automation > Jobs. Find the "Process Email Posts" job. Make a note of its ID. You'll need to schedule this job to run every five minutes. 
6. Make a note of the jobs URL listed at the bottom of the jobs dashboard page. It'll look like this:

https://yourdomain.com/tools/required/jobs?auth=asdlkfjksw8r782348ns...

7. At this point, you can run the process email posts job by going to this URL:

https://yourdomain.com/tools/required/jobs?auth=asdlkfjksw8r782348ns... _JOB_ID_FROM_STEP_5.

8. Using cron or another automated scheduling utility, automate this URL. (Note, cron needs the ampersands escaped so it'll probably look like this:

*/5 * * * * wgethttps://yourdomain.com/tools/required/jobs?auth=asdlkfjksw8r782348ns... _JOB_ID_FROM_STEP_5.

That should be it. Every five minutes the process email job will run, which connects to the email account in question and retrieves messages. When a discussion post is posted, anyone monitoring that forum will get the email message, with the email address as the reply-to address. Reply to that email address and the process email job will pick up that email in five minutes, and route the post to the proper discussion.

1. Go to Dashboard > System & Settings > Mail > Mail Importers.
2. You should see a listing for Discussion Post Reply. Click Edit.
3. Enter information for an email address to use as the reply email address. This is a mailbox that is completely dedicated to this task – something like "discussions-reply@yourdomain.com." You'll have to set this email address up yourself. Enter the information to connect to the email address account (POP/IMAP, etc...)
4. Make sure that the mail importer is "Enabled."
5. Now, head over to Dashboard > System & Settings > Automation > Jobs. Find the "Process Email Posts" job. Make a note of its ID. You'll need to schedule this job to run every five minutes. 
6. Make a note of the jobs URL listed at the bottom of the jobs dashboard page. It'll look like this:

https://yourdomain.com/tools/required/jobs?auth=asdlkfjksw8r782348ns...

7. At this point, you can run the process email posts job by going to this URL:

https://yourdomain.com/tools/required/jobs?auth=asdlkfjksw8r782348ns... _JOB_ID_FROM_STEP_5.

8. Using cron or another automated scheduling utility, automate this URL. (Note, cron needs the ampersands escaped so it'll probably look like this:

*/5 * * * * wgethttps://yourdomain.com/tools/required/jobs?auth=asdlkfjksw8r782348ns... _JOB_ID_FROM_STEP_5.

That should be it. Every five minutes the process email job will run, which connects to the email account in question and retrieves messages. When a discussion post is posted, anyone monitoring that forum will get the email message, with the email address as the reply-to address. Reply to that email address and the process email job will pick up that email in five minutes, and route the post to the proper discussion.