C.O.
Per craigslist.org
craigslist has implemented 2-way email relay to help stop spam and scams.
When replying to a post you'll see an address like:
____@____.com
When answering an email you'll see an address like:
____@____.com
Use your email program as you normally would.
PLEASE NOTE: The “real name” field (e.g. Jane Doe) in your email program is passed through to the recipient. Any contact information in the body of your message will pass through unaltered.