hello 
i´m goind dns funcions , list dns zones  edit records , add mx.domain.com Correct ?
when i try make a test leafdns.com appear 2 wornings 5 faitures  .
Your nameservers did not return any MX records for your domain! This will cause mail servers to use the A record for rainbowdouro.com. as the mail exchanger.
how to fixed these .
Tank you
First of all u need to add a Type A record which u want to use it as mx for your domain.
Example: mail   IN A YOUR_MAIL_IP
Than add Type MX record
Example: MX     5     mail.rainbowdouro.com.
And u can add many MX as u want if u got any backups MX 
Example: MX    10    mail2.otherdomain.com
               MX    15   mail3.otherdomain1.com
But this mail2.otherdomain.com and mail3.otherdomain1.com must exist in there DNZ zone as a Type A record or Cname .