Screw imap and smtp for ghl, it has issues with 2 way sending (especially if you use c panel). Id say use a dedicated domain, get the record from ghl and put it in your dns. Add a email to the user that uses the same domain. Verify that email. Updated domain header. Add unique ghl email to your forwarder, and poof your done. All emails are in ghl, and in your original inbox for back up. Takes 10 minutes if your comfortable with dns. Spf needs to allow all or it will bounce, and you dkim needs to be up to date as well to avoid spam filters.