Mailing attn
You're referring to "Attn" as a shorthand for "Attention"!
When writing a mailing address, "Attn" is often used to get the attention of a specific person or department within an organization. Here's the correct format:
Attn: [Name]
[Organization]
[Address]
[City, State ZIP Code]
For example:
Attn: John Smith
ABC Corporation
123 Main Street
Anytown, CA 12345
Using "Attn" helps ensure that your mail or package is delivered to the correct person or department within the organization.