Namecheap & Mandrill: set up DNS record

add two records in sub-domain settings
this is how it looks like:
domain setting

copy settings from Mandrill website, and paste to Namecheap, just like the image above

futhermore, don't forget to add new domain in Mandrill (reference)

trouble shooting

more than one TXT record that looks like an SPF record.

in Namecheap's mail settings, set to the option to None (Email will not work)

mail settings