Email Sent to customer logic for invoice

Hi Gurus,

I have a doubt on the below requirement.

1. The customer master data there are 3 mails maintained. Please refer the attached screenshot.

2.When i check the output for the invoice of this customer, the mail is always triggered to 2nd id(XYZ@rediffmail.com), if i remove the second email id, then email will be sent to 3rd id(123@yahoo.com) and if i remove 2nd and 3rd email ids, then it is considering the first mail(abc@gmail.com) which is marked as Standard.

I am not able to under the logic behind this. Please help me on this. From where i can see the logic related to this.

Thanks in Advance.

customer-email.png