If you have set in your Preferences to get a notification email when you get a PM message on this forum, be aware of the recipient in the "To:" field of your email reply.
PM messages are designed to be replied in the PM Messages section of the forum where your reply stays private.
If you just click to "Reply" to the notification email, you may be sending it directly back to the Oldgas.com server, not your intended recipient. Look at the "To:" field of your email before you click to Send. Or just make it a practice to reply by clicking on the "My Stuff" link on the Shop Talk forum and choosing "Messages" from the drop-down menu.
Thanks for your cooperation.