Outgoing server dns zone setup help

I am currently using mail.domain.com as the outgoing server. I have a new customer who wants to use smtp.domain.com for there email clients. I added smtp CNAME domain.com. in Main >> DNS Functions >> Edit DNS Zone. When I do a test and send an email, it doesn't work. Can you tell this newbie how to fix this?

Thanks in advance.

 

Oops... after waiting a little while, it worked. I assume due to propagation.