I believe that even though it appears it will show your Cox email address in the From line of your reply, the recipient will see the string of numbers and letters @reply.craigslist.org. This is because your reply isn't going directly to the ad's poster, it is going to craigslist, replacing any personal information, and then forwarded to the poster.
There was a recent change in craigslist handling of email correspondence that somewhat coincided with the Cox email update. The two are not related. The action I described in the first paragraph was always in place, however, in the past it only affected the first reply made to an ad. Any subsequent emails between the ad's poster and replier would in deed show their personal email addresses.
That is no longer the case. I do craigslist a lot and I remember thinking something looked different. Then I came across the notice on the craigslist website where all email exchanges are now routed through craigslist with the sender's email addresses encrypted, as it were.
As a test, place an ad, then reply to yourself. Notice how your From line changes in the Sent and Received emails.
