Mail key

A "mail key" can refer to a few different things, depending on the context. Here are a few possibilities:

1. Mail key (mailing key): In the context of direct mail marketing, a mail key is a unique identifier assigned to each piece of mail sent out. It's used to track the performance of each mail piece, including who received it, when it was delivered, and whether it was opened or responded to.
2. Mail key (postal key): In some countries, a mail key is a special key or code used to access a postal box or mailbox. It's often used to secure the mailbox and prevent unauthorized access.
3. Mail key (password): In some cases, a "mail key" might refer to a password or login credentials used to access a mail account or online mailbox service.

If you could provide more context or clarify which type of "mail key" you're referring to, I'd be happy to try and help you further!