The credit card mailbox is used to accept electronic bills and credit card activities. It was filled in when applying for a credit card. Usually, you don't need to provide a password. Pay attention to your information security.
The email address for receiving the credit card bill is the email address written when handling the card, which can be QQ email address or other email address.
When applying for a credit card number and writing materials, the applicant is required to write personal materials such as email address, mainly to facilitate billing or other credit card-related activities.
If you haven't written or received the email, you can go to the credit card online center of the relevant bank to add the relevant email.