Successfully detected the MX record for subdomain but NOT WORKING

Hi,

Received this email last thursday: Congratulations! Amazon SES has successfully detected the MX record required to use email.mydomain.com as a custom MAIL FROM domain for verified identity mydomain.com in AWS region US East (N. Virginia).

But nearly a week later my emails are still being sent with the following:

mailed-by: amazonses.com
signed-by: mydomain.com

My settings are to use amazonses.com if no MX record is found - so this means that Amazon hasn't found the MX record for email.mydomain.com.

I spoke to my web host (Bluehost) and they told me that the MX record good on their end - they suggest it is on your end.

Please help!

Chris