It's all very well having complete control over the DNS records for your domain, but it allows you to make all sorts of new and interesting mistakes.
Lesson the first. The right hand sign of an MX record should point to an A record. Not a CNAME record. Many modern SMTP clients handle the CNAME situation fine but you run the risk of sending all sorts of interesting error messages back to the ones who don't.
I wondered why my mail load was lighter than usual.
Corrected records are propagating as I type, but if you've sent me something important in the last three or four days and haven't had a response then you might like to resend.