Many forum members have their accounts set up to subscribe to various messages. So when someone replies to a message you get an e-mail with a copy of the message in the e-mail.
Here is the deal: if you reply to the e-mail you are not replying to the forum message. All you are doing is sending your forum message response to me in another e-mail. If you get an e-mail alert about a new post, you need to open the e-mail and click on the "Reply to Post" link at the bottom of the forum message. This takes you to the forum reply window. Do not click on the "reply" button in your e-mail software.
When you send replies to me (firstname.lastname@example.org - the address from which all forum e-mails is sent) your responses are not posted. I cannot copy and paste them into the forum. Thus I have no choice but to delete those e-mails and the replies you think you are making in the forum instead go into the intergalactic bit-bucket.